Snails really arent a big deal tbh. Their population only explodes if youre overfeeding the tank.
Assassin's will eat pond snails quite readily but mts are a different story. Its harder to get at them. No way could they eat a shrimp
A puffer would have an all you can eat buffet. The shrimp and snails would be a tasty snack.
"The simplest explanation for some phenomenon is more likely to be accurate than more complicated explanations." -Occam's razor