We have a community fish tank, and even though the fish experts at the store we go to advised us against it, we have mixed quite a few different types of fishes. The store we go to is strictly a fish store. It is an aquarium of sorts. The employees are knowledgeable in both fresh and salt water fish and the different kinds they carry. We were told for fish tanks to go by the rule one inch per gallon for fish. You have a pleco and those grow fast and I am assuming it is at least 3 inches long. The gouramis are very territorial and when space becomes sparse, they will fight. Adding more fish to the tank will not help the situation. You need to either A) get a bigger tank like a 45 or 55 gallon and not add anymore fish, or B) get rid of some fish. Definitely do not introduce an angel fish if you have aggression problems. They are docile fish but will become aggressive.
We just upgraded to a 125 gallon tank. In it we had originally a pleco (almost 10 inches long) a gold gourami, an angel fish, a blue lobster, a rosy barb, two gold fish (about 3 inches long each). We originally had a 55 gallon. Yesterday we introduced into the tank a blue gourami, a peacock, and a red tail cat fish. We let the fish stay in their bags from the aquarium for about 30 mins while floating in the tank. Our gold gourami checked things out and "felt" the bag with the blue gourami. He uses his long tendrils to feel things. After releasing the new fish into the tank the only issue we had was with our angel fish. She is very dominant but our gold gourami is the protector of the tank and keeps her at bay while he herded the new guys around. They have plenty of room and now the angel fish has settled down.
I definitely suggest you lessen the amount of fish if you are going to have gourami or larger fish. Male gouramis will pick on each other. The way you can tell the sex of the gourami is their top fin. A males top fin will come to a sharp point pointing towards his tail. A female's top fin is rounded and short I will get some pictures up when I get home to show you what I mean.
But definitely lessen the load in the tank, gouramis can be great tank mates but they do become territorial and need room in their tank.