Hey Chris,
I just set up a nano reef with a 4 inch smasher for a buddy, he doesnt harm any of the corals so far. I gave him frags of discosoma,sinularias,etc. All softies though. The only thing he will smash is pretty much anything that enters his layer. Other than that he has been a perfect resident. The biggest problem I can foresee is that he is a tunneler and likes to dig out his layer causing avalanches. My friend just fixes the rock and repositions the coral and its business as usual. If you are considering setting something like this up, I highly recommend it. What a cool little fishtank. He did beat the crap out of a damsel, I was there and witnessed it. Totally cool. I am anti-damsel and it was nice to see them get theirs for once. It was a blue devil. LOL. The mantis popped him twice right in front of us and up floated the damsel.
Mantis shrimp get a bad rap, I think they are fascinating in a species reef tank. My friend feeds him krill,etc. daily. And so far so good on the hermits and snails. No casualties. I bet if you starved him though they would all disappear.
Paul