I have a 55 gallon african tank that has been stable for around 4 years now. The only movement of fish in and out of tanks is for breeding and overpopulation. The first year i lost a few fish to aggression, but i havent lost any in the last 3 years.
As for the key to keeping a stable community - few fish species and lots of each species. Pairs arent going to cut it. Ideally, you want either one of a species or at least five, with as few males as possible (one). The other thing is that they generally do best slightly overcrowded, but with frequent, large water changes and a powerful filter. A long, wide tank is best, as you want more surface area. I would give 25 gallons of water per species if you have groups of them (5 - 7 individuals). Generally, you could also put a single fish of another species in their as well. However, you need at LEAST a 40 gallon tank to house africans at all. For example, i have a 55 gallon, with a single bumblebee cichlid, a single chipokae cichlid, a group of orange zebras, and a group of yellow labs. This mix has worked out well, and follows the guide line i posted earlier. For all africans, lots of rocks are a must