The "Ngoni senior warriors" sculpts are based on drawings of "Angoni" from Nyasaland that the British encountered. That was the predominant style in that area, much as the Watuta figures represent the typical Tuta warrior. But Chris Peers claims both these styles could be found among all Ngoni of the East African region to varying degrees, and not only that, they spread to plenty of people who weren't Ngoni at all.
So I would be perfectly happy to mix them (and I do), or even use them as Sango, Hehe and any number of others.