I agree that the term ''hermit crab'' is synonymous with "reef terrorist". I only have 5 or so in my 75 gal, and they were mostly stowaways on the rock I bought. Even that few though sometimes cause problems like upending frags, killing a few snails (I've lost quite a few margarita snails to the smaller hermits).
However, I give a big +1 on the conch idea. They are very comical looking, almost like a hard-shelled snuffleupagus with their long trunk and cartoonish eyestalks. They're kind of clumsy (my small conch is really good at climbing to the top of the rock, and somehow falling, tumbling all the way to the substrate), but they love detritus and algae. The biggest thing though is that my tongan conch loves bryopsis. He cleared off a rock that was about 4 inches by 4 inches and covered with 2 inch bryopsis in about an hour the other night. A beautiful thing to watch. Anyway, the only downside is that you have to stock the lightly or they may starve. 4 square feet per conch, and you still might need to supplemental feed. I have two in my 75 and so far both seem to be finding adequate food supply.
Anyway, double that and I'm sure you'd be fine to start with. If you notice you need more, add on to it. Just my .02.
