Rams (probably 4 at most), apistos (a pair or a male and several females, depending on species), Bolivian rams (3 or so, maybe 4 - watch out for aggression though), pair of kribs, pair of convicts (in which case no other fish), keyhole cichlids (probably 2 or so), rainbow cichlids. Maybe firemouths, but a 20gal long may be too small. Most of these except the convicts can be kept with community fish. In a 20 gal long you should probably stick to only one cichlid species (definitely for the larger ones listed like kribs and rainbows. Although there is space for pairs of, say, apistos and rams, there may well be territorial aggression).
Also maybe some African Rift Lake species. You could keep several pairs of small shelldwellers in a 20gal long I think.