From: Washington, DC
towerbooks, I am new to this game myself but should know enough to answer your basic questions:
--This is a "hard core" war game and frankly it is not very similar to Civ, and there is no random map creation. While you can create units, sort of, during a game, it has to be rather laboriously scripted via an "Event Editor" when a scenario is created (or existing units can be released, etc.). Generally this game is much more complicated and involved than the other games you mention. You should take a look at some of the AARs in that subforum, should give you a decent idea.
--Players can create their own scenarios, including maps and units, but it is all manual and rather labor-intensive. I am just getting into this now, but the scenario editor looks very flexible and powerful, and you can create games from almost any era with some degree of effort (WWI, WWII, and modern games would be easy, because the units are already included in the database; for others you'd need to create your own units or download some of the player-made unit databases). There is certainly a database for 19th century warfare, and in the scenario sub-forum, you'll see that someone has just created a game about the English Civil War. There might be fantasy DBs as well, not sure.
--I suggest that you start with Advanced Tactics Gold; I haven't played it, but it sounds like more what you are looking for. Not sure how important this is to you, but this game is basically 12 or 15 years old, and the graphics and user-interface are rather archaic. Nonetheless, the game is still being patched and supported, and the most recent patch introduced several nice features. I just bought this game a few weeks ago and so far am rather pleased with it.