hmmm.....
in addition to corydoras of all sorts (for a 20 gallon, I'd encourage you to look into dwarf cories)....you could investigate freshwater gobies. I saw a couple at a LFS and thought they were really cute and interesting.
I think shrimp are terrific, but I dont' think they'll last very long with your frog. Frogs are true predators and will eat anything that moves and that they can fit in their mouths. I had a biology professor in college who used to say "thank heavens frogs body structure doesn't allow them to get much bigger than they are, because we'd all be in trouble."
Loaches are really fun, but be sure to get a species that stays very small (Such as the sidthimonki or chain loach). I have a yoyo loach who's constantly picking through the gravel for scraps.
A note on crayfish.....if you get one, it's going to end up eating the frog or vice-versa.
I agree that you should add to your tetra schools....they are schooling fish and do best in larger groups.....The "black tetras"....are they "Black Skirt Tetras" or "Black Neon Tetras?" I'm guessing they're black skirt tetras, in which case you'll want to add one to three more (with three, it's good to get a one male to two females ratio- if you can't sex them, go with more fish to spread the aggression around-I have three diamond tetras and they school together very happily). You'll want to increase your neons to at least 6. I'd like to suggest that you investigate "cardinal tetras" instead of neons. They're a little less hardy, but I think they're significantly better looking. If you were to add three or four cardinals to your current school of neons, they would all school together.
If you wanted to go with a bit more challenge (but, I think a lot more personality), you might consider investigating
German Blue Ram cichlids (bolivian rams are great too, but may be too big for your set up) or
Shell Dwelling Cichlids (Neolamprologus multifasciatus), which stay very little and live in- you guessed it- sea shells. These guys are all bottom dwellers, so they won't interfere with your mid and upper dwelling fish....
Do you have a LFS that you particularly like? I found this site very helpful in finding an aquarium store (several actually) in my area.
http://www.zoyzoy.com/aquarium/aquariumstoresmap.asp