So if i was thinking of setting up a 55g salt tank in a few years could i start with a brackish first and move the inhabiants over? What brackish fish would work aside from gsp? What marine fish go well with brackish turned salt fish?
Fish like the puffers you mention, Monodactylus, etc. work fine for that purpose. Many marine fish, like damsels, can live in lower salinities (above 1.010, preferably, though) without any issues. Just be sure to raise it very slowly when you do increase the salinity.
Also do a search in this forum as this topic has been brought up a few times. Most of the backish fish that go to salt such as the Monos and Scats will get FAR too large for a 55g in no time. They are active schooling fish that do best in groups of at least 6 and need a 180g. Mollies and Guppies can also go full salt. Hope this helps and good luck!