At that size especially with an asian aro I definitely wouldn't risk it with tank mates yet. They do better with other fish at around 12". But if u must polys make pretty good tank mates. usually fish that don't swim in the same region as the aro (top level). I'd go with datnoids too.
EDIT: DO NOT add a flowerhorn. The FH will most likely beat on the aro eventually if not sooner.