in a 2g, it would probably be better to not ad anything other than the betta, but it probably woulden't hurt to add a small snail, they clean up alot of the food left over, and if your betta is anything like my betta, he won't touch any food that has fallen from the surface, so alot of food ends up on the bottom, in that case, it'd rather have snail poo than rotting food. Although my betta has a 6gal, so adding a snail was not an issue on the bio load really.
Hopefully someone else will come along with more info, I'm definitly not a snail expert, but they seem to like it here, I have probably ~400 of them now LOL. My Puffers can't keep up! LOL