the pencil urchin is not poisonous i believe...the spiny is. Sea urchins in a reaf tank are tricky because they can tople rocks over onto corals. so you need to be sure that they are well secured to prevent this. Also, i think the pencil urchin is omnivor....and is more likely to eat watever is in its path, this may or may not be a good thing. Sea urchins can defenitly be kept in a reef tank as long as certain precautions are taken and also dependant on what species you want.
Sea Goblin