You are mixing fish that should not be mixed together. You need to put your rainbow shark, tiger barb, and angel fish into their own tank. You also need to add at least 4 more tiger barbs. They are a schooling fish and will not behave properly if you don't have a school of them. You need about 29-30G tank for just those fish. Your Swords, black skirt tetra and danio can stay together in a tank. You also need to get four more danios as they are schooling fish as well. Size depends on how many swords you have.
As for the pleco. Odds are its a common pleco that could get up to 24 inches long! If he is a common pleco, he needs to go back to the store unless you have at mini um, a 55G tank to put him in. IMO, 55 is still a bit too small though. I wouldn't go less than 75G but most people say 55G.
As for your gouramis, what kind did you get? If you aren't sure, take a photo. Odds are you have two males and hopefully they are dwarfs. If they are, they can go in with your community fish (swords, danios, tetras, ect)