It will be very nice a native american mod, its a great idea...
*the Lakota-Cheyenne-Arapaho tribes were peaceful tribes with eachother(in Little Big Horn, there was not just Sioux indians fighting against the army, groups of the Arapaho and the Cheyenne joined the fight on the Sioux side), maybe it will bether add Pawnees and Crows as factions, because they were enemies of the other tribes and the white man(But some of the warriors of the 2 tribes accepted join the USA army as scouts helping them exterminate their blood enemies, the Lakota).
btw: I suck at moddeling if you need some research help, i can be usefull
sorry my bad english