look under neith their gills you should see a few "spikes" coming out from the lower part of the gill.
in True perculas these "spikes are usually black and on the ocellaris clownfish they are orange.
hope that helps.
i would pu the hippo in before the yellow and also i have a royal gramma and i wish i never got him because he dosent like anyone new yo be added to the tank and he hates when my cleaner wrasse tries to clean him.