My thoughts are that if you have already detailed culture, religion, economics, foreign relations, and demographics, then the choice should follow very naturally from that.
If you have a feudal culture, then it makes sense for the military to be a feudal army built around a military elite, like...
That is a really good point! It could always be duty, but what happens when some of them start to question that duty? Another possibility: they gain status/prestige and the ability to play political games with human societies. This would make them a great deal more like ancient Mesopotamian...
My advice is to ascertain what, exactly, you want the aboriginal Australian elements to contribute to your West African elements. I phrase it that way because I feel compelled to echo the excellent points made by both WooHooMan and eri about the geographies and levels of social development under...