From what i was always told. You should have at least 1 fish per 5 gals. to help with the aggression issue. In my 75 I have 16 African cichlids, and they have a lil bit of issue with each other but after chasing for so long they lose the other fish. Just make sure you have plenty of cover for them to hide and some will make it there territory. When I was younger and I had my first 55g tank. I mixed all south and african cichlids together and they were fine. But also on my stupidity I had around 20-23 fish in there.