Africans apparently need pretty stringent water conditions, compared CA/SA. Also, they have less personality. I personally don't think most of them are any less aggressive than CA/SA tho. Back 2 months when I first got mine, I had 3 africans (zebra mbuna, livingstoni, kenyi) and 4 south americans (green terror, jack dempsey, convict, texas in the same tank, and the africans certainly held their own.
Due to water conditions they cant be kept in the same tank. Africans need a ph of 8-8.5, while its generally considered "not good" to keep SA/CA in pH that high.