sorry it just seems like your waiting for people to say "oh yeah this fish can go with a jar no prob", but thats not going to happen. some people do keep large jars with other fish, but 99% of the time once a jar gets 8-12" they go nuts and become killers. theres 2 ways to see if a jar will have tankmates, 1. keep putting fish in and if he kills em, he kills em. you might get lucky and find something your jar wont kill, but remember yours is still small and not to the killing stage yet. option 2. have a huge tank, say many thousands of gallons. this way there is room for the jar to have his own territory and give other fish room to run and hide.
i have a small jar too and once he gets bigger i will try and put him with other fish but im not counting on it. i have many tanks so i can keep my jar solitary if need be, and im shure i will have to. the only thing i say can go with a jar no prob is water and food.