Hey Chris that's usually only true for the long spined urchin .there awesome algae eaters and that's all the are . Your safe to keep them in the display if you choose to change your mind . The urchin will do no good in your sump . Keep him up top where he can do his jump and eat up some algae
I have 2 in my 250. Model citizens. They even ate a small patch of bryopsis. Only issue is they like to pick up anything not glued down. Including snails.