Welcome to the forum!
I have a 55 gallon brackish, with one Archer. I have read, that Archers will sometimes fight with their own species. With the potential size of an Archer, I worry that the single, could someday outgrow the tank.
The trick with brackish, is finding fish that are compatible and can live at the same salt level comfortably.
In my brackish tank, I have the Archer, Orange Chromides, Knight Gobies, Mollies, Nerite snails, and a Dragn Goby. (the Dragon Goby also has potential to outgrow the tank)