Yes, it is contagious. While your at your LFS, also get an anti-bacterial med. in case it does turn out to be columnaris.
Your gourami will not have been the one to attack the balas. In fact, I doubt a fish attacking them caused this. It is probably stress. I don't know how big these balas are, but even small ones need a large tank to swim in and they are schooling fish so keeping any number below 3 makes them stressed and nervous (especialy as they are small). Eventualy, you are going to need a 125 gallon minnimum - that's about 500 litres.
A note on the RTBS - they don't like any similar-looking fish. Oftentimes, this includes balas. Once the RTBS is mature, it may become a serious threat in that it will stress the balas out by constantly chasing them around.