Guppies will eat the baby snails. For that matter a guppy will eat anything it can get into it's mouth (typical of most any fish) including dead/dying shrimp and fish. Big fish eats small fish.
Otos are probably about as safe as you're gonna get. I cant see fish like microrasboras and similar being that effective of predators in a well planted tank.
Well, for the record, I keep shrimp with guppies and Endler's, and I can't guarantee they never eat a shrimp, but I've never seen it happen; and I know the shrimp can move fast when they need to (unlike baby snails.)
I have Endlers, Embers, and Pygmy Cory Cats in with my Cherry shrimp. I do not know if they "snack" on the babies, but if they do, it isn't very many of them as the colony of shrimp is booming!!!