Personally, I wouldn't put more than one African in anything less than a 30 gallon. And then I'd overstock the 30 to reduce aggression (maybe 6-8). Depends on the kind of Africans, though. If you have shell dwellers or dwarf cichlids, you would still be overstocked, but I don't think that you would have as many problems down the line as if they were something like demasoni or yellow labs.
Keep in mind, too, that if these fish are juveniles and get along now that doesn't mean it will keep going on. When they get to be adults, they won't get along as well I can tell you that from experience. The success of your mix also depends on your ratio of males to females. Per species it is recommended that you have a ratio of one male to 3-4 females. Or just males of the species. It is very common for the dominant male in the tank to kill off perfectly healthy tankmates over night when the mix of males to females isn't right.
Do you provide caves, rocks, plants, and other hiding places? It is very important that you do...especially with the tank stocked as it is. The hiding spots will at least allow for the subordinate fish to hide from the dominant one.
For now, provide weekly water changes (bi-weekly would probably be better) of at least 25-30%. And research the species of fish that you have and find out if they will work.
And last, but certainly not least:
Just because the fish are survivng, does not mean that they are happy or healthy. We recommend the size of tanks based on the research that we've done and the personal experiences that we've had. I have a 55 gallon with Africans that I've had up and running for almost 2 years now. Keep your fish the way that you want to, but this is what I recommend that you do based on my experiences with these fish.