A pair of flagfish might work but they do prefer cooler temps than most of the other fish you want: Tropical Fish for Freshwater Aquariums: American Flagfish Killifish
Jordanella floridae (American-Flag Fish) — Seriously Fish
What kind of danios were you looking at? Most should work in a 20. Did you do a fish-in cycle or fishless? If fish-in, what fish do you have now or do you have fish that you want to move into the 20 (you mentioned neons)?
What type of substrate do you have? Corys prefer sand but are often kept in gravel as long as it's not sharp and kept very clean.
Without knowing what fish you have now, here's one suggestion:
-- 4-6 panda, pygmy, dwarf, or three lined cory. Most corys would work but I'd avoid the larger types like sterbai for a 20 gal
--6 neon tetra
-- 1 dwarf gourami (would inhabit mostly the upper regions)
--1 bolivian ram (would inhabit mostly the lower regions; only get one as they get aggressive if they mate and other types of rams need higher temps)