My crabs never seem to bother my zoos other than the occasional pass over. If you watch them they are very cautious to tap the zoo lightly to get it to close prior to crawling over them.
Yes, the Hermits are definitely agressive, but not bad enough for me to rid of them. I do see them randomly beating on my snails. Survival of the fittest, and i'll get more if I need.
So far, the only snails I've found missing were some teeny tiny nassarius, but my normal nassarious, cerith, and turbo's do just fine.