I know my Stinkpot would eat any sort of fish that she could possible eat given her size. A significantly larger fish? That can work. Sometimes people put Oscars in with them. (This all depends on your type of turtle)
Snails, though....I'm pretty sure most, if not all turtles would eventually eat them. Unless it was an absolutely massive type of snail...........In that case, please report back with results.
My red eared slider loved the heck out of some ramshorn snails. Just sayin' lol. I've heard of people removing them from their ponds because turtles will even take chunks out of very large fish ( I rescued some painted turtles from a large fishing pond before the guy starting setting up turtle hooks. They were literally hooking fish out of it with bites taken out of them)