I mix old and new world all the time.
I find if they have very different body types they mix especially well, and the Africans can act as dither/target fish.
I always try to keep feeding types together though, example, vegetarians with vegetarians, and carnivores with carnivores etc so I don't run into feeding stress diseases.
And also only combine fish that prefer similar water chemistry, the rift lakes of Africa and the great lakes of Nicaragua are similar, as are Central American rivers.
I would not however, mix African rift lake species with Amazonian soft water species.