I keep old world and new world cichlids together, and it works. Often they ignore each other.
That said, you must choose wisely, you don't say what species of Africans you have, so it's very hard to judge.
Often, cichlids with similar color patterns, or body types don't work, because they see each other as competition.
As to water chemistry, if your water conditions are neutral, and the species have all been bred in your area, it can work.
I have kept S American Acarichthys heckelli in the same tank as L Tanganyika Lepdiolamprologus elongatus with no problem at all. They are very dissimilar in appearance, I believe this is part of the success.
I also keep Madagascans with S and Central Americans, which also seems to work, they are also very dissimilar in appearance.
One that doesn't work is putting the North American Herichthys carpintus, with the Madagascan Paratilapia,
their similar body type, and iridescent spotting made them enemies from the get go, I had to separate within 10 minutes
