You could only keep nearite snails in BW. FW snails will die in there. As long as you raise the SG slowly, over a period of a few weeks, you won't kill off your bacteria. FW SG is 1.000. You'll want to raise your SG between 1.005-8. I don't see any reason you can't keep the puffers you have. When you start adding the marine salt though, I'd get that leaf fish out of there. Actually, I'm surprised the puffers haven't nipped it. Depending on how large the tank is, you could keep knight and/or bumblebee gobies with them.