There are varied opinions about whether or not figure-8 puffers are meant to go in brackish. In the wild, they are occasionally found in slightly salty water. A slow acclimation to saliter water will ensure a good transition.
But you are right about the nipping part - if they get a little curious about another fish (which can happen quite a bit), you might see chunks of fins (or worse) missing. Given enough space and/or retreats, the problem tends to be lessened.
However, a 30 gal will not work long term for the fat sleeper, if you're sure you've got that species. They get way too large for that size tank, and you'll likely have to upgrade soon. Rather get a larger tank and then move into brackish with him.