Since you asked--these probably aren't the best, but I've enjoyed the Calabim and played them quite often, so I might as well give my opinions.
1. Religion. I haven't really played Elijah, so I can't comment on him here, but in my personal experience, the leader has a strong bearing on religion. Flauros and Order synergize very well. Since Order doesn't make you good in RifE, you can run it along with Slavery, letting you mine your hills for extra hammers. You never want to use Unyielding Order, because that will remove the unhappiness and kill your production from Governors' Manors. Instead, stack high garrisons along with Social Order to push up your happy cap and you get one hammer for each population as well as the benefit of that population.
For Alexis, on the other hand, I favor RoK. With philosophical and a lack of access to Elder Councils, getting an early Great Prophet is very logical, and he can bulb RoK while you research more useful techs. Arete is both good for production and for even more Great People--after getting a second prophet for the shrine, you want to go for engineers. This allows you to get Mines quickly, and then start building Master buildings for more gold. Though you'll never have as much commerce as Flauros, this approach will give you, along with RoK itself--probably running God King, see #2--will give you pretty much unlimited gold so that you can keep the science slider high. Furthermore, you can skip smelting/iron working for a long time because of the iron from the mines and the fact that vampires come at Feudalism instead of Iron Working. I like to imagine Alexis convincing her people that she is the avatar of their Goddess this way. =P
2. Civics Like with religion, the Calabim have two somewhat divergent civic preferences, in my opinion, based on the leader. For Flauros, Aristocracy is somewhat of am obvious choice, because Financial interacts strongly with it, and you can take Agrarianism along with this to make each farm productive as well as worth nontrivial commerce. Of course, this leads to slower growth, but you won't be getting vampires as quickly anyway. However, sanitation is a strong priority with this system, for the extra food from farms as well as for sewers. Personally, I always settle river cities for the fresh water anyway, but this incentivizes it even further. In low hammer, high food cities, the whipping aspect of slavery is still strong as well.
With Alexis on the other hand, I strongly prefer God King (God Queen?) early on, moving to Republic later for the extra great person boost to stack with the others and the extra traits. She can make good use of these, considering that Aggressive can be a bit weak at times. But, since you won't be using Aristocracy, the boost to cottages from Taxation is relatively strong anyway. The lack of the aristocracy food malus to farms combines well with the fact that you'll have your iron vampires out earlier and can more freely feast them. I personally favor foreign trade for her, but you could make a strong argument for Conquest since you'll have food-heavy cities and you'll want Warfare anyway for Training Yards and National Epic (for even more GPP).
3. Magic. Despite the removal of death promotions from vampires (well-deserved nerf, imo), the Calabim are one of the strongest magic civilizations. Of course, it goes without saying that one vampire in every stack should have body 2 for regen, but it goes well beyond that. Starting with body and shadow mana means that even an adept can make an early stack vastly more powerful, and potentially negate the need to carry along clunky siege. If you do need siege, then you definitely want to consider a Force adept so that your catapults aren't slowing down your stacks, but of course a mage or two with Shadowwalk and fireballs completely negates the need for any siege. Shadowwalk actually does work--pay attention to a city's defense % if you have a Shadowwalk'd unit selected versus one that isn't. And, of course, once you hit archmagi, you have the best casters in the world because they can feast their way to insane exp and and probably the only civ with a nontrivial chance at Omnipotence.
You should probably grab at least one Earth node--between all those mines and farms, the resource discovery is fairly nontrivial, and it also allows you to defend cities more easily. It should also be noted that Law mana does allow for a cheap summon that makes great cannon fodder that takes no additional investment since you have a law node already. Especially for Alexis, it can be worthwhile to train a squad of mages with just law and spell extension/combat promotions.
4. Conquest. This is perhaps more of a warning, but be aware of the cost that comes with conquest. Though with the religious choices I've proposed above, both main leaders have a strong counter for it, you're more vulnerable to this cost than many civs simply because of the reduced effectiveness of Governors' Manors as compared to normal courthouses. Of course, you can feast down captured cities, but remember that the amount of exp you get from feasting is proportional to the city population. Also, both conquest and agrarianism will deny you foreign trade routes.
5. Food Stored After Growth. There are more than a few buildings that store some percentage of food after growth. The obvious ones are Granary, Smokehouse, and Breeding Pit, but you also have Salthouse and Growth (the creation I spell--creation mana is always a good investment, at least for long enough to get a few mages with creation I, and one with creation II since health is so useful when population is hammers, hammers, and exp). If I remember correctly, all give of these together should store 100% of food after growth, allowing you to regrow population very quickly. This is a significant boost to feasting and slavery, as well as simply growing up to your happycap if you get new resources or other sources of happiness. In addition, three of these are bonus health, and Breeding Pit is +2 food with no malus if you build a sewer in the city.
6. Vassals. Since Feudalism is an important tech for you anyway, take advantage of it to take vassals. Protracted wars of conquest are tiresome, and vassals are generally more valuable than their last few cities. Demanding palace mana from a thoroughly defeated vassal is a great way to get more without much investment, and you get trade routes with vassals even if you're running a civic that doesn't allow for foreign trade routes. Plus, what self-respecting vampire doesn't enjoy having a few pretty slavegirls (or handsome slaveboys, as the case may be) to play with while mastering the world?
7. Religion Revisited. In addition to (and despite) the specific advice above, there are definite benefits to spreading non-state religions if you aren't going for a religious victory. Especially for Alexis, who can, if she wants, run religion in Cultural Values, temples from extra religions are more happy citizens to offset the penalties from slavery and feasting, and the specific benefits of Order, RoK and Empyrean temples are often valuable even when not following that religion. And, if you end up going on a conquest, the odds are you'll pick up a second shrine at some point. It can also be beneficial to found Esus yourself, for Nox and the sight it grants you as founder.
8. Undercouncil. Like any good Evil civ, the Calabim can make excellent use of the Undercouncil. In addition to meeting civilizations you may not be geographically close to, it offers you a number of beneficial resolutions and effectively no negative ones. The diplomatic bonus with other council members isn't strong enough that you need to worry if you decide you no longer want Secret Codes in place, while the free great sage from it can be quite powerful. The Undercouncil is not so much an alliance as a tool for exploiting others, and the Calabim are well equipped to make use of it.
9. Allies. Good civilizations often make better allies than Evil ones. While the initial malus for being evil can be painful at first, the bonus for sustained peace tends to outweigh it, which few very Evil civilizations will give you. Beyond that, especially as Alexis with the RoK route, you can generally find a good civilization who will convert to your religion and very possibly keep it. Both Order and RoK will pull you and your potential ally toward Lawful, which is a nontrivial bonus when a Chaotic Evil leader will often hate you as much for being Lawful as a Lawful Good leader will hate you for being Evil. Also, you don't really favor death or entropy mana, which will avoid those maluses from Good civilizations. Being Evil, after all, is not so much a matter of faith as it is for, say, the Sheaim--you're evil because the world sees what you do as evil, not because you have some strong ideological conviction. That makes it far easier to convince a potential ally that you are a "necessary evil," rather than an abomination to destroy at all costs. And since you naturally oppose Armageddon as much as they do, it can be entirely convenient if they should desire your aid in some petty conflict with the Sheaim or the the Illians or the Donivello.
10. Terraform. Back to magic here, but you have much more to gain than your rivals by ensuring that your lands are perfectly suited to you. Scorch and Spring together, along with Vitalize and Drought later, allow you to create a perfect paradise wherever you desire, and to tailor the output of your lands to your needs. Desert and tundra and swamp alike succumb to the power of your magics, and so you need not hesitate in the face of them if you wish to seize an otherwise exceptional location. In many cases, you will favor all grassland, but plains hills can be useful. Or, if you should happen to share borders with, say, the Hippus, creating an artificial desert along the boundary of your land can ensure that they cannot so quickly swarm across. It should be noted that Stonewardens can go a step further for Alexis, turning any tile into a hill. Windmills can be used to spread irrigation, if you find or create a hill in a place that would otherwise impede your farming.