Just to reiterate: you do not need to, and ideally shouldn't, keep glassfish in brackish water. They are FRESHWATER fish that in some cases coming from blackwater streams where the water is very soft and acidic! Frank Schaefer described then rather nicely in the Aqualog book as "brackish water fishes that aren't". Sure, they'll tolerate brackish water conditions, but they certainly don't need them, and will live perfectly well in good quality freshwater.
They aren't like mollies, violet gobies, figure-8 puffers, etc. that are sold as freshwater fish but rarely do well in such. Those fish genuinely need brackish water to do well.
You can keep glassfish with most anything. They're very efficient predators and will eat small fish including livebearer fry, and one of my specimens (unfortunately) ate a juvenile carp about the size of a neon tetra. They'd likely eat shrimps too. But apart from that, they're good community fish if a bit boisterous. Mine live well alongside halfbeaks, South American puffers, irrubesco puffers, diamond tetras, bleeding heart tetras and various catfish.
I agree 100% with three-fingers above re: painted glassfish. Do not buy them. Not only is it absolutely barbaric how they paint them, it also makes the glassfish dramatically more prone to the viral disease lymphocystis. What's the point of getting a luminous green glassfish if it's going to be covered with tumours for months or years? Plain vanilla glassfish are infinitely better fish. They're feisty, hanging about in gangs chasing each other about. They live for several years and provided they're feeding are basically hardy, low-maintenance fish that make a great alternative to tetras or barbs.
Cheers, Neale