I would Keep the aro in the tank for a few weeks before introducing any other fish. This way the aro will be the more aggresive fish.
This worked for me when I had 3 aro's together. I had the smallest one (6") in there for a few weeks and then introduced the other two (12" & 22")
It was sooooo funny watching the smallest one chasing the other two around the tank.
They were getting along fine, but one jumped out and died. The good news is, I have a 2 aro community with no problems.