Can we get a little more information from you to help with answering your question? How big is your aquarium? And how long have the other fish been in this aquarium? What color is your rainbowfish?
Some of the rainbow species can be a bit bullyish, especially the males. My intuition is telling me that you purchased a red rainbow and that if that is the case, he probably will not bother your gouramis. But I really need more info in order to make more than just a guess.