Oh there's too many of species that could keep in 20gal. So you need to tell us what kind set of tank, water temp, breeding purposes, colors and community tank? Some species are not suited for community tank. Some species need to be in cooler temp if they want to reproduce. Some species will eat their own fry.
List of species are suited for 20gal:
Least killifish (heterandria formosa)
Tiger Teddy (neoherandria elegans)
Gold Teddy
Humpback limia
Sulfur limia
Blackbelly limia
Tiger limia
Cuban limia
Few limia species
Endlers
Wild Guppy
Eastern Mosquitofish (gambusia holbrooki)
Gambusia species
Blacksided Mosquitofish (flexipenis vittatus)
Brachys species
One spot molly (poecilia vivapara) (sp?)
Amazon molly
Orangetailed goodeid
Characodon species (few localities are suited to 20gal)
Ilyodon spp
Skiffa spp
Few wild xiphophorus species (im no expert on them)
Poeciliopsis spp
Carlhubbissa (spell?)
Probably more species that I can't thought of.