With Brackish aquariums, the fish can handle swings in SG as they naturally occur in estuarys and other zones that fluctuate regularly.
Your filter bacteria however, needs more time to adapt or you risk wiping them out and starting a mini cycle (like when you set the tank up) this can cause the loss of fish pretty quickly and lead to nuisance algae etc.
I remember being told (and I have always done this) to only adjust the SG by 0.002 SG every 2 weeks. any more than that and you risk causing a crash and another cycle.
I checked out your pictures in the other thread and that light colored puffer looks like an oddball figure 8 with an interesting pattern, a very nice one actually.
Guppies and mollies do exceptionally well in brackish water (mollies are native to it). I have even acclimated cheap mollies to full marine conditions, that lived long happy lives and reproduced. In fact those mollies were with a GSP in full marine conditions.
So livebearers are a very good choice for a tankmate and puffers will mostly ignore them.
Pristella tetras can handle low levels of salinity and any of the indian barb species from the dawkinsia group.
here are my Filamentosa barbs that I've had for about 2 years now. they are in freshwater but they make for active colorful additions and stay at a nice medium size.
males in breeding dress