I wouldnt plan keeping a flowerhorn in a 55 for life ESPECIALLY if your not completely positive its a shortbody even then i personally wouldnt but i guess it could be done. My FH is 13" now by himself in a 75, i wish i had gone bigger with the tank though.
Also as a general rule as stated there really is no such thing as a non agressive FH. Heck mine has always killed anything i tried to put in there with him. He killed a 7-8" pleco when he was just 3" and beat the **** out of any other cichlids i added even much larger than himself til i finally gave up, they are definately solo fish lol.