A longed spines urchin would not be the best addition to a reef tank. They are an eating machine when it comes to algaes including your prized coralline. Their rather large and cumbersome spines can also damage corals and knock things over.
If you are set on an urchin, I would instead suggest a blue tuxedo Mespilia globulus. They are not as cumbersome, reef friendly and will not ""strip mine"" the coralline (that rhymed). I have had one now for a few years and it has not given me any reason for concern.
Cheers
Steve