Emeralds don't bother SPS. The lifetime diet of emeralds is still a subject that needs a lot of research on it, however I've got a 120g SPS tank and I have 3 emeralds, and I never see them near my SPS while they're out at night. I do see them eat algae all the time, so I'm pretty confident that they're doing their job.
I've never been a fan of hermits because in my nano they decimated many astreas in the hunt for a larger shell. I've now got only one blueleg in either of my tanks, and he's in my 120, and how he got there I suppose I will never know, as I didn't put him there. He's good though and I see him grazing on rocks most of the time, however he does harass snails and climbs on corals occasionally. I've heard mixed reviews on sallylightfoots as well. I personally don't trust crabs anymore. I'm very cautious of my 3 emeralds.
