Space is your main consideration obviously, as others have mentioned. For a 20g, you are (or should be
) looking at "dwarf" type species among the Cyprinids or Characins. These are not near so commonly seen as the "standard fare" and include the ember Tetra, Celestial Pearl Danio, Mosquito Rasbora (and several other species in Boraras). There are some "larger" species that do well in a 20g because they are not active swimmers. Any of the pencilfish (species in Nannostomus) in the Characins would come into this category. Require soft water; and Ember Tetra work very well with these, colourful and interesting.
Cichlids are out, except for a pair or trio of one of the dwarf South American species (Apistogramma, Dicrossus genera) if water is soft; if hard, the Lake Tanganyika shell dwellers. Quite different. But plants would be limited, some Corkscrew Vallisneria, a sand substrate, some rocks would make an authentic aquascape.
Gourami would have to be restricted to either the Sparkling gourami (a group of 5) in soft water; or a trio of Honey Gourami is basic water. Companions to these from among the rasbora, one of the dwarf species, or one of the three "Harlequin" type in Trigonostigma. There are some other suitable gourami, but they are not easy fish, and with no intention to disrespect, they take some experience.
Substrate fish for bottom interest, depending upon the upper fish and water params, might include corys, Whiptail Catfish, BristleNose Pleco.