I've been reading a lot of cichlid-specific sites for stocking suggestions and I keep seeing the same basic idea- 15-25 fish for a 55 gallon tank. Now mine is a 50 gallon so I would cut this back a little bit, but surely with my current level of five cichlids and one plecostamus I'm not overstocked. Here is one example that I pulled off of a cichlid site, but you can find many with similar numbers:
1 trio Protomelas
1 trio Aulonocara
1 trio Copadichromis
1 trio Labidochromis or Cynotilapia
4 Neolamprologus (larger species)
2 Eretmodus or 1 trio Haplochromis
2 Julidochromis (larger species)
1 Synodontis Catfish
1 medium sized Plecostomus
It seems to be the common wisdom with cichlids that overstocking will moderate their aggression. Of course you need good filtration, but my powerhead filters the water over 10 times per hour, plus the sponge filter and two box filters. Another common theme I see on cichlid-specific forums is that there is no such thing as too much filtration for cichlids. Also I am doing regular tests for PH and nitrites- I still can not seem to find an ammonia test anywhere, and the shop that said they would order one did not come through!