If you're staying with South America, not the gourami. But any of the dwarf cichlids (one species only though) will work. Some are best in pairs (male/female), some in a harem (1 male/2-3 females). Most will need very soft acidic water (as does the rummynose)since they will be wild caught, or perhaps tank raised from a local breeder. One of my favs is Dicrossus filamentosus
or Dicrossus maculatus
. These are absolutely stunning fish. Most of the Apistogramma, but they are not all "the same" with respect to behaviours, size. The Bolivian Ram
is nice, I would suggest either a single male or a bonded pair in that sized tank--and not any "pair" will work, they have to decide and bond. Same for the common ram Mikrogeophagus ramirezi
. The latter needs warmth, fine for the rummys though pushing it a bit and esp for the Lemons.
And of course substrate fish; there are several smaller/medium catfish that will work singly or some in groups.