Bumblebee gobies are indeed a brackishwater fish. I hope you realize this and have them in your tank accordingly! However, I have read reports of them being kept successfully, even breeding in freshwater. This would not be easy however and so I reccommend keeping them in lightly salted brackish water. Your best bet for a puffer tankmate in those water conditions is the figure 8 puffer, they might be too aggressive with the tiny little bumblebee's though, and eventually they will need a slightly larger tank. Dwarf Puffers are likely the best bet to live with bumblebee's because of their size, but you would need to have the bumblebee's is freshwater which would almost certainly prove difficult at best.
Maybe post a new thread with a request from nmonks about this, he has some knowlege about keeping the Bumblebee Gobie in freshwater.
Good Luck
SLC