Hey EJKC, first of all what kind of aro do you have? Although many people say that there is a different level of aggression in arowanas (silvers being more accomodating and australians being evil bastards), arowanas in general are very territorial. I'm sure there are aro's with more aggression or less agression, but in general finding tankmates for your aro is more or less an experiment.
I got my australian aro when it was ~4-5" and also bought a ornate bichir ~5" at the same time. At first the bichir was the one that got really aggressive during feeding time and was nipping at aussie's tail fin; tore it up a little a couple of times. However, now that aussie is 7-8" the bichir is the one that is under attack. The bichir doesn't swim around the tank as much and I've seen aussie patroling the tank and keeping the bichir in check. So far no deaths, but I'm still hoping I don't wake up one morning and finding the bichir floating lifeless.
I've heard of people trying to keep aro's together in a tank, but I think that unless you have a pack of them (5+) that they tend to end up fighting each other to the death. I'm sure that you don't want your RTG and your other aro to start maulling each other.
Other tankmates that may be compatible with aro's are tiger datnoids or loaches, or in general fish that are bigger than your aro and ones that tend to swim at the bottom of the tank. Hope this is helpful, and also could you post a pic of your RTG. I'm so jealous that you have one!