All the hermits will kill snails for their shells, if you don't have any shells available for them. Scarlets are better than most for NOT killing snails.
I would stick with any of the hermits that stay very small. Any hermit that gets bigger than an inch or so will normally 'change behavior' and become a pest, and blue legs and scarlets are better than most for being 'good' crabs. You can also try haloween hermits if you can afford them - they are awesome looking hermits.
Emerald crabs are excellent for the reef, they love hair algae. I have found that they can get rather large and strong too, and will knock over stuff not secured well.
Any crab that has red eyes should be avoided, as a rule (even if tiny) they will eat corals.
As a general rule, all the crabs fall into the 'watch closely' catagory with the only exception the filter feeding sandcrabs and porcelan crabs - these will have net-like mouth parts and will never pick on corals or fish.
If I were you, I would ditch the larger hermits at the LFS
for something else. I have also seen larger hermits kill and eat smaller ones as well as any fish they can catch.