Sometimes GT's are to aggressive for oscars. Oscars are more mellow. A BP is fine with an oscar. I have an oscar and a BP together and they get along great. GT's are mighty aggressive and it will only get worse. BUT if you want to keep the oscar and PB you need a 120 gallon. If you want to keep just the GT he will be fine in the 55 gallon. You will be very over stocked. For now I would do 50% water change once a week.