I have four Earthmovers (3, 5" Satanoperca jurupari and 1, 3.5" Geophagus braziliensis) in with a 26" Silver Aro. BUT, they are in a very large vat (625 gallon) with lots of hiding places AND the Aro never was given live food and has not ever eaten any of his tankmates (with the possible exception of some black tetras, but other fish may have been responsible). Just keep the Aro fed well and provide some hides. One other thing, I had several other moderately aggressive cichlid speices like Severums and Mexican Rainbows in with the Earthmovers and generally everything leaves them alone. Not saying, however, a really mean species wouldn't go after them.
Starting to think that Arowanas are not the sharpest tools in the shed, mine tried several times to eat my magnetic algae scraper!!