20 longs are great for corydoras or kuhli loaches!
Top:
Honey Gourami (2F, 1M)
Middle:
Endler's Livebearers (lots-these guys are tiny but colorful! 2F for every 1M)
OR
Harlequin Rasboras (6-8)
OR
Cherry Barbs (6, 2F for every 1M)
Bottom:
8-10 kuhli loaches
OR
6-8 corydoras (bronze/peppered/albino,panda)
If you like smaller fish... habrosus or hastatus corydoras max out just under 1" and you could have about 14-16 in there! :drool:
Edit: Also, your bottom dwellers would appreciate a sand substrate, but as long as your gravel doesn't have sharp edges it should be fine. Gourami need some floating plants (real or fake).