The thing is, africans are way more aggressive than convicts. A convict wouldn't stand a chance against an african. But.. Africans will do fine if kept with their own. You can easily keep 20 africans in a 75g tank if you provide the proper environment for them. Lots of rocks, good filtration, and a high quality food.
I don't care much for a lot of the south/central american cichlids just because you're so limited in how many you can have. Oscars, for example, get so large and are so aggressive, there are only a few fish you can put with them. Having 2 or 3 fish in a 75g tank just looks boring to me. It's all personal preference though. If you haven't already, check out cichlid-forum.com and go to their profiles section. They have info on just about every cichlid that has been discovered to date. Lots of good info in there.