The Order
The Empyrean
The Runes of Kilmorph
The Fellowship of the Leaves
The Octopus Overlords
The Council of Esus
The Ashen Veil
That looks like 7 religions, not 6.
The Order, Empyrean, Runes of Kilmorph, and, Council of Esus worship single gods (Junil, Lugus, and Kilmorph, repectivey), but the Fellowship of the Leaves, Octopus Overlords, and Ashen Veil do not. The Fellowship worship nature in genera rather than a god. Of course they consider the God of Nature quite important but they revere the God of Life (who was the original God of Nature), God of Death, and God of Fertility too. The cults of the overlords follow dread creatures ff the abyss created from the dreams of a man who is the dream of the sleeping God of Water under the influence of the servant of the God of Mind. Danalin would be worshiped indirectly, but they know very little of his true nature and many followers have probably never heard of him. Mammon is just about as important to this faith, although this god doesn't like his followers to know about him. The Ashen Veil serves a pandemonium headed by Agares, but which reveres all the evil gods too, especially Bhall, Aeron, and Ceridwen. Agares prefers his demons to compete for his affections, and so does not place them in a set hierarchy. As such, I tend to think that the Ashen Veil isn't so much a unified, organized religions as a group of allied cults, which may be devoted to or make deals with rival demon lords. Different gods are probably emphasized by different groups within the faith.
Just like in vanilla Civ IV, 7 of the most prominent world religions were chosen even though other religions do exit too in the game's respective worlds. Firaxis considered 7 the best number for balance reasons and to fit the normal size maps. Kael pretty much just went along with this reasoning, although before the Shadow phase he had only implemented 5 (well, technically 6, but the Cult of the Dragon could not be adopted, could only be founded by 2 civs, and lacked all the trappings of most religions).
I believe there was some discussion of adding more religions but only letting a maximum of 7 religions be present in any one game. The mutually exclusive religions would have been those of the same alignment. That idea never really went though though, mostly because the team wants all the religions to be unique and things get more repetitive when you try to add more.
We do know some things about the other gods' religions:
The Emyrs worship Ceridwen, and are a powerful cult in the Sheaim lands. They excel at arcane magic, have access to all kinds of obscure lore, and are devoted to destroying the world. I tend to think this sect may have close ties to the Ashen Veil, possibly even being seen as an elite group within the larger religion.
The Stewards of Inequity worship Mammon, but only those of the highest rank in this secret society know this. Mammon prefers his followers to think they serve only their own interests. The group is organized similar to the Freemasons, and use similar symbols. They are like the Illuminati or other supposed secret conspiracies. They are allied with the Council of Esus, but are far less violent. Most members only really care about their own coffers. This group is dedicated to keeping the rich rich and the poor poor, not letting little things like hard work get in the way of the distribution of resources. Members can be found among merchants, bankers, and especially tax collectors in any society, but they are most common among the Balseraphs. They appear in the game in a counterfeiting event.
The Druids are the priests of Sucellus, and are often but not always associated with the Fellowship of the Leaves. Followers of Sucellus in general are called The Untouched, and are known to be very powerful healers.
Sirona doesn't really have an organized religion per se, but there are several groups founded in her name to provide charitable relief to the less fortunate. Henri Ghouls (who would later be adopted by Kylorin and become Perpentach) was raised in an orphanage run the the Sisterhod of Sirona back in the and of Magic. Immanuel Logos was originally a priest-king of Sirona, but he abandoned the position to found the Order of the Elohim, an order of monks devoted to caring for the orphans of the godswar regardless their religious beliefs. Sirona approved of this decision, and considered it actually serving her even more. Sirona is still highly revered by the Elohim, but those faithful to her are more concerned with helping the helpless than with proselytizing.
Tali has no organized religion, but many adventurers maintain shrines in his honor. These are usually placed in hard to reach places, like the edges of really high and remote cliffs. There may be some smaller shrines of the steeply sloped roofs of some inns too. Followers of Tali tend to be daredevils who do things just because they seem cool, and who have almost no attention span (at least for anything useful).
Camulos, God of Chaos, also has no organized religion but does have shrines. One of these is founded by an even in game. His followers tend to be sociopaths who worship him by causing others needless pain, although some prefer self mutilation. He is most popular among violent despots, insane generals, and the Doviello.
Amathaon has no organized religion, and praying to him doesn't do much good. He seems to choose who to bless with talents haphazardly. Some worshipers of Amathaon tithe to those who display clear genius, but this may be more like showing their appreciation to the artistic community than actual worship.
Arawn is revered by many among the Sidar and a few among other societies. However, the god of death is completely indifferent to their worship, and the utter lack of miracles or responses of any kind to prayers offered to him shows. Even the Sidar have given up on maintaining a priesthood to him.
Those who follow Oghma tend to be scholars and librarians. I don't think he has a real organized religion though. His vault has been invaded by demons loyal to Mammon, so he and his angels are too busy to deal with mortals on Erebus.
Bhall used to be by far the most popular deity, before her fall. The Clan of Embers remain fanatically devoted to Bhall, but the other races turned away from her when her priests went crazy during the fire rain. She would however still be an important figure in the Ashen Veil. In some ways she is more important here than Agares, as without her passion the demon lords in the lowest hell are fully given over to despair and don't have the will to carry out thie rschemes on Erebus.
I don't think Nantosuelta has a religion. She is concerned with only intrinsic, not positional, worth, so she would probably have no desire to see mortals bow down and worship her. Those who revere her would probably be more philosophical than religious, concerned with trying to become a better person and just enjoying life. She is however called the angel of faith, and her followers would be what you call spiritual although not religious. As Nantosuelta loves Sucellus, she may be revered by the Fellowship fo the Leaves and by the Untouched.
Aeron used to have a cult/guild devoted to him in the game, Aeron's bounty. This was an association for assassins and sociopaths. Aeron's vault is also what could be called the greatest temple of the Ashen Veil, where processed souls go to learn to become demon lords. I suspect that most mortals who revere Aeron however see religion as a tool to use to dominate weaker beings, and so would likely not really believe whatever religion they profess.
Dagda has no religion.
The Luonnatar worship the one true god, and reject all the angels/gods are petulant children. They are right, but this seems to just be a lucky guess. They have no supernatural ability, although their faith protects them from the supernatural. (Originally the story that justifies their magic immunity is was just explained as Junil not wanting to hurt them for fear that their presence meant The One had returned and was unhappy, but I prefer to think that The Compact protects them from any god, angel, or demon.)
