If you keep the tank the way it is, the goldfish, pleco, and 3 sharks will be severly stunted. This means their outsides stop growing because there isn't enough room, but their insides(organs, etc) keep growing. This shortens their life span considerably and is very painful for the fish.
Think about it. Would you like to be kept in a box barely larger than you? You wouldn't get any bigger and your insides would damaged. Why should the fish have to go through this torture?
If the sharks keep growing, they will eat your smaller fish. I suggest returning the goldies, pleco, and shark and just having a nice community tank. You don't want to cruel to these fish, do you?