My only concern would be that some bettas might harass them and nip at their antennae thingys. Depends on the betta's personality, though; I have one that totally ignores the snail and another that I wouldn't dare add a snail (or any other living creature) with him. An oto is another option, but that's kinda like the snail - may go badly if it's a naughty/aggressive betta.
Hmmm, I can't think of that one snail that's totally hidden under it's shell, but it could eat all it wanted without being harassed. Maybe someone else can think of it?
