My jar is cool with bottom dwellers - rays, bichirs, catfish. What you would want is mid/top level swimmers to go with it. Since it is still small and if you have the opportunity - i would suggest go with more jardini (5 or more if budget permits you). this way, they get socialise at a young stage. I would be nice to see a colony of jars swimming on a display tank.
Since ive got my poor man's arowana mature already, it had been a trial and error. my first attempt was with some more jars - even with the same size, the black jar, managed to damage and kill them - shredded bodies got eaten by the bottom dwellers. It's an expensive and risky attempt. Second try i'm doing now is with bulky americans. I have a blackbelt, red devil and oscars surviving and I think, i'll be loosing a jaguar in a couple of days.
It's still your decision and what EVZ said, they are notorious psychos. Very nice to have though, specially if real asian arowana's are impossible to get hold on.