i have a four inch oscar, and two pink convicts in a grow out tank. i have reconsider my plans, and i dont think im going to be keeping the oscar, but man i would love to. i think once arond six inches im going to give him to a lfs. anyway, the convicts are about a inch and a half, and are breeding even, and dont seem to mess with the oscar too much. they stay in their area, and since the oscar is bigger than them theres not much problems. i would say though, in a 20 gallon, if the convicts breed ( which is imminent if there is a m/f combination) and the oscar is not much if at all bigger than the convicts, yuo might run into some problems. also, you said your 3 year old oscar is only 8 inches?? is it common for oscars to range in size from 8 to 16 inches!!? that is a huge range. i would assume the oscar is full grown at 3 years as well.