when dealing with cichlids you want to look at the tank dimensions, filtration, and the adult size of the fish you want to keep. with a 28 gallon i dont think you have too many options, maybe some type of shelly.
Shellies would work, so would Rams , Apistos' , Dwarf Acaras, and just about any thing that stays under 6". With the larger ones(4-6" fish) you would do best to keep only a single specimen or at most a pair. Any more and your looking for trouble.