Red claw crabs in quarantine

[Seahorse man]

I just set up a small 5 gallon tank and am thinking about getting a red claw crab or two and giving them a island, but also being able to house a quarantined fish in there if things turned south. Will red claw crabs live long in full scale saltwater? Or should I just abandon that idea?

 

[DSC reef]

I don't know much about them but from what I've read they should be kept in brackish, not a full marine environment. I've also read they are to be kept singly. How about a scarlet hermit or Halloween hermit?