I have 7 urchins in my 215 reef, most for over 2 years. They scrape off the coralline algae, but then excrete its spores, so that it grows back fast as well. I have 2 blue tuxes, 2 black long-spines (with a zebra crab that keeps them from getting too big), a purple "rock boring" urchin, an "impact urchin", and a pink/red pin cushion. They can knock things over, but I've gotten better at planning against that - I also have big snails and sea stars that can do that. I haven't seen them cause any real trouble. Camo means camouflage, and that means that small rocks, debris, and even snails can get picked up and carried around on the urchin.