I can't offer much info about longspine urchins. If you use the search function you should be able to find out some more. AFAIK one the biggest issues w/ them is knocking things over-especially as they get bigger. Not the best choice for smaller tanks w/ coral. You'll also want to be sure your rock work is stable to avoid a rock slide if you get one. They also consume coralline, which may be an issue w/ you. Be cautious when handling them or working in a tank w/ them as well.
I'm not very familiar w/ the hosting behaviors of cardinals, but they may only host in the spines for protection as juveniles. Besides not be necessary in a captive reef, it may also grow out of it. HTH