Steve, I think the Diadema urchins are most likely to eat Valonia, but they grow way too huge way too fast to go in to my 90 (or the 20). I did add a Tuxedo urchin to my 90 to see if he would help at all, and I don't think he eats anything other than coralline. An urchin in my Zoa tank would be a disaster since nothing is glued down in there!
Which urchin are you thinking of in particular?