Used to have a 10" Silver Arowana (very peaceful) and I tried to house it with my 6" RTG. First two weeks was good until the RTG gained a bit of momentum and almost beat the silver to death. Then I made my decision to just keep the RTG, who just simply isn't afraid of anything (which include my hands when feeding) and I traded in my silver. It takes trial and error and is a big risk to take when trying to house multiple arowanas although like aeri said, just have a spare tank just in case they don't co-exist or maybe a tank divider if you still want to give it a shot, and yes, individual fish personality is another huge factor.