If your aro hasnt shown any signs of aggression I would go for it. Mine wont bother anything that wont easily fit in its mouth. Usually they are fairly passive.
My Plec makes my Oscar cower in a corner, it's HILARIOUS. The O deserves is cuz he kinda bullies most of the others, but it's more of just a show. He doesn't pay the least bit of attention the knifes, which is what keeps him from being segregated (at least til their growth warrants the next move).
Ive, got my 13" CK with my 13" aro and they have no problems. The aro is king of the top and the CK chills out at the bottom. Even during feeding they are not aggresive towards each other.