It is true what Tihsho is saying (by the way is your name supposed to say "tissue"? haha).
Dying prematurely is simply them dying sooner than their normal life expectancy. Keeping brackish water fish in freshwater can shorten their lifespan (there has been talk that a bit of salt in the freshwater improves the health and longevity of freshwater fish, but that is not 100% confirmed and most likely dependent on species), and vice versa. Same can be said with keeping fish in undesirable water parameters (temp, TDS, pH, etc.). How much the lifespan is shortened depends on many factors.
As mentioned there are many species of bumble bee gobies, some which really do need brackish water. It is possible you got a freshwater species, research how to ID your species. Fish can be tolerant (some more than others, some even completely adaptable) of salinity, so it is tough to say how much the lifespan is shortened, but if there are many reports of brackish gobies dying quick in freshwater that is something to take note of (many factors, such as severity of water param differences, overall fish health, etc play a role).
And I also do agree with what Tihsho has said about the puffers as he is mentioning what he finds is best for the well being of the fish. That is his advice and experience with dwarf puffers and I agree. Not entirely sure about the group thing, the are social fish, but in groups they can quarrel, but I suppose that is natural behavior and the fish themselves are used to and fine with it and actually prefer those interactions (even if they may not look pleasant) rather than kept singly. Now if one is being overstressed/harassed, then actions should be taken before the stressed fish dies (either more hiding/retreat spots or more females, or maybe another male so the males can spread out the aggression. But my experience on dwarf puffers is limited, but what knowledge I have gained, most if not all of what Tihsho has said seems true.
But alas, all we can do is offer advice and suggestions on more ideal/preferred set ups.