i got a pincushion urchcin a whiel ago for my reef, and here is my little note: (lol)
although urchins do not naturaly eat corals, they do scrape their teeth across anything they can come in contact with, even if they are corals. usualy softies and such arent hurt, but if it is somethign hard like an sps that they could actualy climb on, dammage is possible. Also, in an attempt to camoflauge, they will pick up all sorts of thigns and bulldoze all sorts of others, including any corals taht are not firmly attached. i had a lot of sps frags that were either look on a disk or wedged into a rock, and they were all picked up, scattered all around, left in the sand, behind my rockwork, and knocked over of left in places so they were touching, IE, death or at least temp recession on all cases.
just something to watch out for