in my fiance's 55 gallon, she has a 10" albino oscar and I have my 10" jar in there as a grow out. There are also 3 Jade gobys, a raphael catfish and a upside down cat in there. The jar started out in a 20 gallon when I first got him at 4". That night, I bought two little catfish and put them in with him....20 minutes later, both catfish were dead so he lived a few months solo.
then I took a jump and threw him in the 55 gallon. He is harmless to any of the fish. I personally think it made a difference that I put him in there when he was 7", and he was the smallest fish. Now he is the same size as the oscar and even 4" bigger then my smallest jade goby, and they get along great. I Havent seen him even nip at anyone in about 6 months.
now, in my other tank, I have a 21" silver. I put the Oscar in there with him and the oscar had a dime sized piece of flesh hanging off his skull in 2 seconds flat.