Tuxedo urchins are about as sharp as a hairbrush. You can safely pick them up. As for compatibility, they have absolutely no interest in anything that isn't algae or a small item they can wear. Their single potential compatibility issue (aside from big fish that might eat them) is that they carry small things around for protection and camouflage, and may pick up small coral frags, hermit crabs, or snails. Nothing eats them, at least nothing you would be likely to have in your tank, because they're sea urchins.