It all depends on of the hippo will be able to weather the storm, your yellow tang will probably beat on it for a while to let it know who is boss...after a few days he should leave it alone. To help keep the hippo tang healthy, have a cleaner shrimp to clean his wounds and be ready to feed heavily, as the ensuing onslaught will probably result in an ich outbreak in the hippo, I see this regardless if I quarantine or not. I have introduced tangs in with tangs, and that is what I have experienced. If you see a fight starting and separate the fish, when you reintroduce the fight will start again. Another option is remove your yellow for a few days while the hippo gets used to the tank, then reintroduce the yellow and the fighting will be less but still there.