Frogs are really good at cleaning the bottom of the tank like a pleco, just not the sides.
African dwarf frogs stay very small too and are usually out of the way anyways.
You have to be careful as a lot of pet stores mis-label them as a clawed frog or vice-versa.
Here's a site on how to tell the difference:
http://allaboutfrogs.org/info/specie...edordwarf.html

The clawed frogs will actually eat the africans and a lot of other small fish. And they get much larger.

They do have fish that are similar to the plecos, but I'm not sure on their size or compatability with a betta.
BUT a lot of pet stores offer a return policy for when the pleco outgrows your tank. IF I don't have a bigger tank by then & I can't find a home for them I am returning them to my pet store when they get too big, and trade for smaller ones again.
And they do "kind of" grow to their environment but they will still be too big for a small tank once full grown.