You are right about them. In my opinion, they are definitely not community fish. You are also probably right about them looking nice and suckering you in. You aren't the first one to buy fish without researching first and certainly won't be the last. If you can return them, that would be the best way to go but if not, you can usually increase their numbers to spread the aggression.
Also, from looking at your other fish, you will definitely need to start looking for a larger tank for the pleco and the cihlids. The pleco will grow up to 2 feet long and grow pretty quickly. He will probably outgrow your tank in less than a year. They are also terribly messy fish so they cut down on your stocking level.
The cichlids will reach about 5 inches but I'm not sure how fast they grow. Having 4 could also be a problem as I think they are best kept as a single fish or in a male/female pair. The info I found said a single pair would need at least a 40 gallon tank and that it ws possible to keep 2 pair in a 55 gallon tank if properly decorated. You may want to ask about them in the New World Cichlid section.