Drawf gouramies and bettas are for the most part are almost never aggressive, but, they, along with angels and rams, would be a very poor selection for a tank with ranbows, becuase of the activity level of ranbows, and will most likely have trouble feeding. Most tetras (and other similar fish) will be scared into hiding, so they make poor tankmates for ranbows as well. Yor best bet would be to go with 2-3 more rainbows, and 1-2 more corys, and then your done, and you won't have any incompatibility issues.