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.