apple snails too big for a 2 gal. they grow to the size of a golf ball
id stick to some "pest" snails or trumpet snails. will not over run the tank if you dont over feed. just make sure the snails get some food too. if feeding only pellets the snails may starve. a small pinch of flake food (and i mean small-just make sure the betta doesnt eat it all) should be just fine to supply the snails with what they need.
all the best!