I have tried mystery snails w/ bettas with poor results. I "surrendered" 2 of mine to the local Petco when I discovered how much they poop!!! The 3rd smaller snail died (see my recent post on betta Care). Bettas love to provoke big snails. Bettas eat the tiny pond snails and any young snails small enough to fit in their mouths. I've made it a habit to go to the local Petco and ask if I can pick the baby pond snails out of the plant tanks- they're free, and the bettas eat them up. I'm hoping to have snails that will eventually colonize and provide food + tank cleaners.
4 bettas: Butch, 3-year-old violet male PK EE & 3 girls in a 30 gal sorority. I do not name the girls anymore! 1/2 of them die within a month.
30 gal: 9 Oto cats, 4 Ocellifer cats. Inverts: Cherry shrimp & 2 assasin snails.
10 gal.: Butch, his 5 oto cats, 3 assasin snails, and a few snails the assassins haven't caught.