If they are the electric blue hermits that I am thinking of (with blue and black striped legs), they are reef safe. I currently have about 6 of them in my tank. One word of caution however, they seem to be the most aggressive of the hermits in my tank, ie., they ate 6-10 of my snails. I am now leaning more towards a snail only (for clean-up critters). And yes, they get REALLY big - about 2x's bigger than my biggest scarlet; one is currently inhabiting a huge shell previously occupied by big Turbo.
HTH - Jared.
P.S. A good site for purchace and info on clean-up critters is: http://www.reeftopia.com/