what type of an aro are you eyeing for???? for a silver aro, a tank 5 x 2 x 2 can house it for a year. once the silver breach the 12" mark, there is a tendency of biting tail in a narrow tank.... no need for a tank mate in a tank that size!
I would get a small ray for a bottom dweller and a knife or a datniod or even a pbass for a middle fish and I would also consider getting a bigger tank down the line
Do the bigger more aggressive cichlids (Jaguar, Festae, ect.) not cause problems for an Arowana? Are they able to live their full lives co-existing in the same tank?