Is there a way (via XML) to mix up the units a bit, rather than being forced into having 3 clones of the game trooper per unit?

Like what if I wanted to do up a unit that had two Pikemen to either side of a single Knight?

is it done through formations?

Should be possible - you will need to look into formations (Units\Civ4FormationsInfos.xml) - Settler might be a good place to start looking for a example - those mixed settler units are composed of different figures from separate nif's - should be possible to replicate that on other units.

And: Sticky: Reminder: QUESTIONS BELONG IN THE MAIN CREATION FORUM! (http://forums.civfanatics.com/showthread.php?t=163616)
http://img389.imageshack.us/img389/8571/angrymobns9.gif