tuxedo urchins are fine in pony tanks. I'd just keep away from the diadema or other real spiny guys.. just because ponies are clumsy and don't tend to look where they are hitching.

FWIW.. I've kept scarlet, blue, zebra, all kinds of hermit crabs in my pony tank without ill effect. Yeah, every once in a while they try to get some "tail" but the ponies never get bitten. The darned asterina stars are more of a nuisance to the seahorses than the hermit crabs.

Dave