In addition, I wouldn't suggest going out and purchasing a BBG. But if this is a matter of "I've got one, now what?", then let me say I have had success with a single BBG and a single GSP kept in a 55g tank at 1.008sg. That said, I have no intention of keeping them together like this forever. The BBG and GSP leave each other alone, however the GSP is still a juvenile.
Of course, that's just me, and I've been [knock on wood] lucky [/knock].
Since you seem to have the ultimate responsibility of separating the puffers, I would take care of that first, then look into including BBGs in your low-end brackish tank.