Just a few notes that I figured I'd put down about modding civs. Some of you will know this already, but hopefully it'll be useful for someone
Active Season: Normally, you have Elves in the Spring, Humans in the Summer, and Dwarves in the Fall. You can have additional races show up for trading by giving them the AUTUMN and WINTER seasons. You will get another race show up with the dwarves to trade in the fall. Yes, two can trade at exactly the same time, the game gives you the option of which merchants you want to trade with when you say trade.
I've also discovered that races will go to very long distances to trade. I had a race I'd set up to only live in the tundra and glaciers and set to trade in the winter. I'd have them show up to trade even in the tropics. Long trek.
Evil races: You can only have one active. The BABYSNATCHER tag has them both try to snatch children and send out ambush parties. If you do not have this tag, they will just start sieging as soon as they'd normally start sending in baby snatchers. Adds an interesting challenge to the game and makes defense a priority. I've had a siege as early as the first winter. Thank goodness for locked doors.
Giving an evil race the AMBUSHER tag will make them do a somewhat sneaky siege. You still get the message saying a force of vile darkness has arrived, but they start off hidden. So you don't get any advance warning of where they are. I'm not sure yet if they will walk to your fort without being discovered or not. I do know that once you find them, they march on your fort and attack.
[ March 31, 2008: Message edited by: Mephansteras ]