Bumblebees can be kept in freshwater, but many aquarists have found them easier to keep in slightly brackish water. Whether they need the hardness and alkalinity, or the actual salt, is unknown: in the wild, they live primarily in freshwater not brackish. I have kept them in soft/acid water, but they did not do as well as I'd expect, so at the very least, they need hard/alkaline water.
If you're just keeping these gobies, I'd recommend adding a little salt (1-2 g per litre) which would allow you to keep plenty of plants. These gobies need plants to do well, because they are very territorial and like to "perch" on things. Java fern and java moss would do well and don't need much light. Add a few olive nerites or Amano shrimp to crop back the algae. Malayan livebearing snails do well in brackish, too.
Bumblebees should be kept at a gallon or more per fish. In a 7 gallon, you could probably keep 7 fish easily, provided the tank had lots of plants and some caves. When kept in groups they are very active, and chase one another around the tank all the time.
Whilst you could probably keep dwarf puffers at this low salinity, I don't know anyone who has. I'd tend to stick with the gobies and, if you wanted something else, perhaps add a few Heterandria formosa. These are nice, small livebearers that aren't much kept but well worth the effort.
Cheers,
Neale
