It's best to stick with about 3 species in a 55 gallon. Then about 5 each, with 1 male and 4 females. Yellow labs ( Labidochromis Caeruleus), rusties (Iodotropheus sprengerae), and acei (Pseudotropheus acei) make great additions. These three are generally less aggressive and still nice looking. If you want you can add some bottom dwellers like Synodontis Petricola, of some other syno catfish species. Really it's a matter of what you want. When I set up my 55 gallon I picked a list of species I liked then decided on the best mix. Now I have a very colorful display that fun to watch and easy to care for. You want to generall avoid Kenyis (Pseudotropheus lombardoi), buble bees (Pseudotropheus crabro) and golden cichlids( Melanochromis auratus). These three species are generally very aggressive and have been known to go about killing tankmates.