This is one of those situations where it depends on the personality of the fish. Plenty of people have Jars that go psycho and kill everything else in the tank. I'm one of those lucky people who have a Jar that doesn't really care about the other fish. Mine's in a mixed community tank with cichlids, clown loaches etc and while the Jar may occasionally chase one of the other fish briefly he doesn't attack them for real.
It seems bottom dwellers like bichirs or tough fish like big plecos are generally amongst the most successful tankmates for jars - but be prepared to re-home the Jar quickly if it goes nuts. If you're not able to move the Jar pretty much straight away if things go wrong it might be too risky.