For the most part they leave crabs alone like wicked said... Mine will occasionally nip at them in my FO tank but he hasnt succedded yet!
Lionfish i have seen in a reef tank as well. They wont eat crabs necessarily but they will go after your shrimp and perhaps other small fish that will fit in its mouth. It should be added last if you were to put one in there. But, remember lion fish have poisionous fins and can sting you if you go in and add another coral... I wouldnt recommend them but it is possible.