It can be done, I have had my 16"-18" Jar with a lot of tank mates without to much trouble. Including 2"-3" clown loachs, green terrors, jags, bichirs, catfish..... probably more I just can't remember. Out of all those he killed only one and 90% of his tank mates he really just ignores, unless they try to take his food. That is a different story
When I was looking for a peaceful Jar, I would look at any 12" or bigger animal at my lfs that had a good record with tank mates. I was surprised to see how many were out there! There was a 2 foot fish that had lived at one of my lfs for at least a year with dozens of different fish ( mostly silvers) and never killed anything!
There were so many in my area(three nice ones), at the time, that I actually began to be picky. The reason I picked the Jar that I did, besides the fact he had a good track record with other fish, was because he had great colour.
If you are serious about finding a peaceful jar you have to put in some man hours and look for 12" and above animals that have a good track record, they are out there you just have to be patient.