I haven't kept an aro but have kept many balas, I think that the only problem for a while will be the hyperactive balas making the aro a bit spooky. In a large enough tank it should get used to it.
i just bought a bala shark about 3 in and i put it with my jardini.. the jardini looks like it wants to eat it but it havent yet.. well see what happens its only been one day.