Most rainbowfish are freshwater, not brackish fish. However, Pseudomugil cyanodorsalis are brackish rainbows (blue-eyes to be more specific) and very pretty little fish, if you can find them.
Don't do that to a freshwater rainbow like a bosemani. Yes, they might survive because they're tough fish, but they won't do as well and their colors won't be good. Almost all rainbows that you can find available are freshwater, and should stay that way. And it's not just me that is saying this. The world renowned fish discoverer and rainbow expert Heiko Bleher himself made this point very clearly on a rainbowfish forum a few months ago. Bleher is the one that discovered and brought the first praecox rainbows, Bleher's rainbows, turquoise rainbows and many others from the wild. If you have one of these rainbows, it's likely a descendant of the fish he brought back as very few other people have been able to successfully bring live rainbows out of the wild jungles of New Guinea. There are a few brackish rainbows, mostly blue eyes species like P. cyanodorsalis, but not many.There is a number of brackish rainbowfish once you start looking into it, as well as freshwater species (such as bosemani) that can be acclimated to brackish.