I apologize for sounding upset, but I am, and i hope you understand why. In my 10gal heavily planted tank, I originally bought 4 mystery snails, all decent sized. After reading on these forums that they're okay to be housed with bettas (and seeing tanks with both in there) I thought itd be okay too. Well the first betta I got completely tormented them, and nipped off half of their tentacles before I could remove him from the tank. He would literally hover above them and nip and them despite me tapping on the tank to shoo him away. I returned him to try a different betta, maybe I just got an aggressive one. I got a smaller betta too, as the first one was pretty big. Same thing happened, once the new one got warmed up. He was okay in beginning after I fed him, but in a little time he started attacking them too. My poor snails barely have any feelers left besides one of them.
On top of all that, I look like a fool in front of the Petco people (lol) since they even warned me it probably wouldn't work after I returned the first one. I said no its okay I see other people who have them! Well now I'm going to have to go back with this one, can't wait for that. I guess I'll just get some cory catfish and not get any betta, the reason I spent time on this NPT in the first place.
Unless your temperature is low and which will make your betta not that active, or hes being overfed like a pig, I can't imagine bettas getting along with mystery snails.