i keep an arowana with my peacock bass..
i got my Dozer when he was 2" and when he was about 4" i brought home Pinky, my aro, she was about 6" at the time. Now Pinky is close to 2 ft, and Dozer was 16". They lived together every day since Pinky came home.
Oscars work too, they grow about the same rate, but oscars will gang up on other fish in the tank, so it's best to only have one. i made the mistake of buying 2.. now i have 2 tanks with a single oscar in with peacock bass.