I recently took a break from fish, but had been keeping African and South American Cichlids since I was 14. The best way I was able to sustain my 55 gallon tank was with deep aragonite sand and crushed coral mixed, tons of rock hiding spots, 2 canister filters (these fish produced a lot of waste so over filtration was important), and 2 power heads (one with a venturi to pump air in the water). Every time I "overstocked" the tank it ended in a lot of dead fish. I personally had better luck picking similar type cichlids and stocking the tank when they were all the same size. Anytime they got aggressive I would move the rocks around to remove territorial aggression.