For Great Generals, you can attach a general to a tile with multiple units, and they will not only get an experience boost (the 20xp shared amongst them), but I *believe* that it will also completely heal them instantly. I have to say that I have never done that, so please correct me if I'm wrong and I'll edit this post
Dunno where you heard that, but as far as I know, that's not been true under either version (Wrlds/BtS) or any patch. It only grants XP, and if that gets a unit a promotion, then it will heal normally with that promotion.
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I have heard that Woodsman 3 and Medic promotions stack if they're on the same unit, but not if they're on different units. Haven't gotten the chance to try it yet, but that'd be awesome, I think.
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I'm not totally in love with colonies, and I think a little mor work needs to be done on them. Essentially, a colony become a vassal with a +10 attitude adjustment that never seems to go away. I believe that this diplo bonus should melt away like most other diplo effects do, so that at some point, if you haven't been treating them nicely, they might decide to leave you; why should they be any different than anything else?
I have also noticed that making colonies is not always to your benefit, and you must do so intelligently. If you're just grabbing a bunch of land and not putting a lot into it, you will be spending a fortune to pay for that far off land while you develop it, so making a colony and gifting to it some extra cities you grabbed land for will save you a lot of money in the short term. The downside is that it's a very weak nation that can't help you for a long time, and may fall victim to your enemy if you don't defend it.
OTOH, if you've developed these cities with banks and courthouses and all that, then you will take a serious hit if you free them. Not only have you invested a ton of time and money and effort into making these cities a productive part of your empire, but now your income and science and production take a hit, and if you're dropping 10 cities, the increase in the time it takes to get your next techs can become substantial. How much your income changes will depend on the pervasiveness of corps in the colony's cities.
Colonies SHOULD get the international trade bonus, and definitely get the overseas bonus. I have noticed that having single-city islands does wonders for my trade on my main land mass even under Mercantilism, but there are so many possible bonii I can't remember which is which or why each comes up. 3.13 made a lot of changes to things too, I'm sure.
As to your Warlord problem, you could use them defensively, but yeah, I agree that they seem to die too easily. My suggested solution would be to give the Warlord an automatic 90% retreat chance, and have the Tactics promotion apply to the stack. Any other retreat bonii would be divided by 10 and added to the 90%, so a Warlord with Tactics would have a 93% retreat chance, for instance. (I don't think this would allow a 100% chance for any unit, but we could get awfully close. Maybe subs.)