My 55 gallon community tank has 11 zebra danios with the following tankmates:
1 Dwarf Gourami
15 Neon and Cardinal Tetras
7 Rummynose Tetras (MY FAVORITE!)
8 Harlequin Rasboras
5 Sunset Platties (1 male, 4 females)
2 Golden Platties (1 male, 1 female)
8 Julii Corydoras
4 Swordtails (1 male, 3 females)
Some Ghost and Red Cherry Shrimp, here and there.
And a couple Ramshorn Snails
They all seem to be living pretty well together. For a 20 gallon setup, I would suggest Neon Tetras, they're relatively cheap (especially during the sales at Petsmart in Silverdale, when they sell tetras for a dollar each......it's not the BEST sale out there, but around here, it's the best you can get. If you want to drive down to Tacoma, there's a pet store at B&I that sell Neon Tetras for like 50 cents each, or something like that. Totally worth the trip, I'd say.)