I'm not really sure about Cossacks, but I got over 2000 places during an 8 player game. I haven't actually filled all the slots up though, but building another barracks/town hall etc. did raise it. So at least if there was a pop cap it is well beyond that. I actually never even used over 1 000 soldiers though. Now C2 is supposed to have a pop cap of 64 000 (possibly split up among players). I must admit that I never got that far, but I do get over 1 000 units quite often.
And that horse thing wasn't the only thing wrong with Rome, but also the unlocking of factions is something that I don't like, it doesn't fit in with a PC game, or even an RTS in general. And Rome was pretty unbalanced (Germans couldn't even get a decent wall), and in historical battles I couldn't figure out how to play the other guy. I also had problems with breaking down the palace in cities I conquered, not that I had to do that too much, but sometimes it was the final level of palace and I had to replace it with one of my own, but couldn't upgrade it nor could I downgrade it in order to upgrade it with my own. Same thing with farms, and neither could I set a rating of what was sold and what was meant for growth. And these are just examples.