Sounds like a nice-sized setup, and a P. volitans should do well in a tank that size.
Since you're getting the lionfish, you'll need to plan your stocking list around it, as there are some considerations in that regard. The lionfish will become a 12" - 15" cube of fish in terms of displacement (this is counting finnage), and they do have large mouths, so anything they can swallow pretty much WILL be swallowed (fish, crustaceans, altho hermits are OK).
A CUC of assorted snails and scarlet reef hermits (Paguristes cadenati) works fine with lionfish. They pay no attention to sea stars, but starfish tend to do poorly in tanks with elevated NO3 levels, which most predator tanks have. Cleaner shrimp are hit-or-miss...sometimes they get a "pass", sometimes not (best bet is to add the shrimp first, or let it go into the rockwork after lights out).
Fish groups to avoid with lionfishes:
Large-species angelfishes
Non-planktonic-feeding triggerfishes (Xanthichthys and Melicthys spp are exceptions)
Puffers
These fishes will pick at and harass the lionfish relentlessly, and the lion usually dies as a result.
Fishes that do well with lionfishes:
Lionfishes! All of our DT's contain multiple lionfish species.
Larger scorpionfishes and stingfishes.
Soldierfishes/Squirrelfishes
Large wrasses (harlequin tuskfish are a good one)
Marine bettas
Less aggressive eels
That's a partial list...if you have any questions, give a shout out.
