Brackish salinity is usually around 1.012. Fluctuation to 1.008 or 1.014 will not bother your fish as long as the change is slow. Hydrometers are cheap($7) and probably the only extra piece of equipment you need.
Archers do get big. You would definitely need a bigger tank. As a side not ,alot of archer tanks are filled two thirds of the way to the top with branches sticking out of the water. This is in an attempt to get them to display their archer skills by shooting insects off the branch. Anyway, puffers are very hit or miss in terms of aggression. I have seen huge mbu puffer in planted tanks with rainbowfish and tetras, but no one is going to recommend this. Your puffer won't be in any danger, but the archerfish may get nipped. You can always try, just keep a good watch as puffers can go for awhile being passive and then become agressive as they mature. There really are no easy solutions for tankmates with a puffer. Most people would say keep them as solitary fish. I keep mine with a green spotted puffer and they have been together for 6 months, so you never know. Good luck!