[garfieldnfish]

A bristlenose can get too rowdy for the frogs, rather then the other way around. But what works well is African Dwarf Frogs and clown plecos. Clowns also stay small, eat primarily wood (needs driftwood), algae, veggies and ripe mangos, so there is no competition over the frog food (bloodworms and frog bites). Frogs are slow in finding their food so this is important. Both clown plecos and frogs like a pondlike setting with slow moving water, driftwood and live plants. I have lucky bamboo in my tank with some crypts, valisneria and a sword plant and some java moss (but that does not seem to do so well in this tank compared to the stuff in my other ones.)
If you want to add a beta be careful. This may work if the beta is a friendly one, but he could just as well turn out to take the frogs limps off one by one. A female beta would be a better choice. In a 20 gal you can also add Singapore wood shrimp or amano shrimp with great success. Small tetras (glolights, von rios, lampeyes, rummy nose etc.) instead of the beta may be a better choice. While they will eat some of the frogs bloodworms, this will not hurt them and once the bloodworms hit the gravel they tend to ignore them and this gives the frogs a chance to get their food.

20 gal my recommendation:

2 clown plecos 6"
3 ADFs 4.5"
2 wood shrimp 1" (some pleople don't count shrimp at all since they reduce the waste rather then contribute to it)
6 small tetras 9"
(or just one clown pleco and 8 tetras, but I always think unless they are likely to kill each other in close confines any animal will do better if it has another of it's kind with it. Clown plecos are totally non-aggressive and tolerate each other very well even in a small tank)
= 20.5"