I have a pair of common short spined urchins that came in as hitch hikers on my live rock. They are in the FO--banned from the reef. They tend to be little bulldozers, and will knock corals off as the move around. Otherwise, these guys are good grazers and help spread coralline algae around. Research the species before purchase though--some are obligate feeders on sponges, tough to support.
Stars--it really depends. The brittle stars tend to be fairly easy to care for, but they must be target fed--there is not enough leftovers in an average tank to keep one alive. I have a red one that was a hitch hiker (a pic is posted in the hitch hiker thread). It's about doubled in size in the past 6 months. Just hangs around a coral, grabs food when we offer it, and looks scary.
Some species are more prone to hunting if not fed, but they are not tough to care for. Serpent stars are similar--a good choice. Chocolate chips are predatory, as are most knobby stars--not good in a reef, but fine in FO. The linkia stars are are very tough to get in good condition, and not for the beginner (or most aquariasts, if survival rates are the appropriate guide).