Nickfish doesnt have a ton of experience, so he doesnt realize that a sfe eel can indeed gulp down a shrimp, considering it is natural prey. BUT, they wotn eat any hard shelled crustacean. They will eat those other crabs you see, like the procelains and the emeralds, etc. But they wont go after snails or hermits, although im not sure about conchs...
So, yea, u can keep hermits and snails, an eel wont just swallow a shell like that, cuz they'd never digest it, and they know that. On the other hand, the kinds of crabs that they CAN eat, they can crunch down on em and make em digestable.