A 20 long will limit you to a single species if you're thinking pairs (start with 4-5 and see how they're doing after a pair forms - you might need to remove some or all except for the pair), but you could have 2-3 single specimens of different species if you'd prefer that - oh, and we're talking dwarf species here. For example:
M. altispinosa (Bolivian rams), M. ramerezi (Blue/Gold/German ram), any Laetacara (dorsigera & curviceps are the most common), Keyholes (not sure I'm up to spelling their scientific name - biggest fish listed so far, but very peaceful), N. anomola (golden eye dwarf cichlid - have lots of hiding places for the much bigger male if you have a pair - females with fry are remarkably aggressive), dwarf pikes, Apistogrammas (lots of choices - borelli, cockatoo, etc. - most are harem breeders so one male and 2 female would do well in a 20 long), etc.
Take a look at the profiles on this site and see what catches your fancy.
All the above do best with a nice dither tetra school up top. My favorites are bolivian rams and the dwarf acaras (the keyholes and Laetacaras) because I find them to be dwarf cichlids that are outgoing and tough (i.e. - they don't know they're considered dwarfs) yet not actively aggressive. For example, my large male L. dorsigera bosses around two angel fish right now - not aggressive, but he gets first choice of foods, space, etc.
Eric