I have hermits and snails in with my porcg puffer. He loves to stalk them. I've had to replace some every few months as he likes his live snacks. Plus they are good for his beak.
I have 4 hermits with my Porcupine puffer..they are small. the very first day he tried to eat the little one and it would go far enough into the shell and he couldnt get it. Now he knows that I am his source of food. he doesnt even pay any mind to the hermits even when they are walking in plain view.
I had a couple of small red legged hermits with about a 6 inch dog faced. He never touched them. Every fish is different, but if yours does fancy the taste of hermits, like stated above it is very good for his beak to bite at the shells.