The tribes specifically have the closest thing to a Eurocentric culture, yes; though they trade all across the Silken Sea when possible, and dwell on the same contiguous land form as both the Nhirians (more Iranian/Mesopotamian) and the Atalmo (an Afro slaver-society) as well. So depending on the proximity of your tribe to these cultures, and their relationships to you, you have opportunities for quite a bit of ethnic blending, whether through trade, interbreeding, or hegemony. You're making me imagine something like Haiti happening in the east and that's very cool, as I have drawn almost entirely from Old World sources.

If it helps,remember that the Angaturiz are one of two ethnic groups who the "civilized," imperial nations believe make the best slaves. So if you want your tribe to have the most possible ethnic diversity then you should probably represent one of the eastern tribes who the Nhirians have already suppressed; or a coastal tribe, one regularly raided for fresh bodies.

Edit: You can play people from other nations as well, although they won't be chieftains, for obvious reasons, and you'll need a strong backstory explaining why they're in tribal lands.