The betta would not eat the corys, but the shrimp might be at risk, especially when they molt! Keep in mind though, if you get the pygmy corys, you would probably not want to section off the tank later, that wouldn't leave them with enough room. I really like the snail I have, it used to be with my betta, the snail was about the size of a quarter or so, and the betta never bothered it