Do not bother with snails. Clawed frogs will eat small snails I am all but sure and trapdoor snails can be dangerous, they can recoil trapping a clawed frogs arm easily and like Ally stated there is documentation (even pictures) of this happening to frogs.

It's not worth the risk. I thought about adding ramshorn snails to my tank but I decided I do not want to introduce something possibly dangerous into the tank. Snails are known for carrying a lot of parasites that infect vertebrates.