I forgot yesterday, so before I forget again...welcome to TFF.
First thing, the lone Rainbow...this is what occurs when shoaling fish are in too small a group, or here, alone. Shoaling fish inherently have an expectation of being in a group of their own. The reasons they need this are several, but without getting into those, I will just say that this is programmed into the fish's DNA and we cannot change it. The fish "expects" the shoal, and becomes frustrated when denied. A scientific study carried out a couple of years back proved that increased aggression was the normal response from a fish when numbers were not sufficient. So, point is, this is nothing out of the ordinary. You have two options, either remove the rainbow or increase the group to 6-7 minimum. Now, this can create another problem; once a fish becomes more aggressive due to this frustration, it does not usually reverse if conditions are then corrected; it depends upon the individual fish and how it was affected. So the safest thing may be to remove the rainbow. If more are added, this one might be very aggressive towards them, and you only increase the problem.
Second, on the gourami and store advice. The store was incorrect. Gourami are sedate fish, all species of them, and they do not appreciate active fish around them. Here again it is something unnatural to the fish, and because the fish is not programmed to deal with this, it is more stress. Stress causes 95% of all fish disease, so avoiding as much stress as possible will always mean healthier fish. I have studied fish for many years, and you will not find any reliable ichthyologists or biologists who disagree with this assessment.
Store staff are often unfortunately not that knowledgeable, to varying degrees. Just read several threads on this forum that include problems with advice from stores. One person once told me of a store where she worked, and all staff had to take lessons in fish husbandry before they could be hired; this is certainly the exception, not the norm.
Byron.