I'm glad to see this topic come up; I just started trying to mod the mod a bit. Firstly, I set all of the minor/unplayable civs to playable by humans. This worked out just fine. However, I soon came across problems:
I'm hoping to mod some leaders (specifically the "war chief" generic leader for different North American tribes), and thought I had it down; changed the Apache CivilizationInfos.xml leader to GERONIMO and added a GERONIMO leader to the LeaderHeadInfos.xml. In addition, I edited the NORTH_NATIVE leader's traits. None of this has had any effect, save eliminating any option for an Apache leader. The others are still called War Chief, and have the same traits as before -cultural and expansive (which, by the way, makes no sense to me whatsoever). The biggest problem for me is that I cannot even find where War Chief data is located, since all I really want to do is edit that data.