I keep a Jar with 4 silvers, 3 blacks and an asian. The Jar is the smallest of them all, but fears no one...she acts tough but without a problem...( i've kept this sort of community for years and years..)
I have, however, another Jar who has killed more fish, some biger than her, than I care to remember.
I have tried her with an FRT, the thing went stressy for two days and now it is allright.
So allright that I added in a Papyrocranus Afer ( african knifefish, some call it the Arowana knife )
They are the same size, measure each other like to toughs, but the experience is working....