Calm has nothing to do with it lol. If someone give you a plate of food you don't eat it because you are agressive, you do so because you are hungry >_>. The MTS are more at risk than the ramshorns, if the beta can fit the snail in its mouth he can crunch it up and eat it.
The rule of thumb with fish: if x will fit in y fish's mouth, consider x food.
I was just teasing, but in all honesty....I wouldn't be worried if my Betta chowed down on some icky pest snails. You don't realize it, but the plants you put in probably had some egg clutches somewhere on them. I did a bleach dip on my plants and an egg sac survived it. Hope you are lucky though and don't get over run.