I have never owned a sun catfish, but I do currently own two black fin sharks. They kind of look the same but different colors. But what I was wanting to say is that, when they get bigger they will probably eat smaller fish. Yes they are peacefully, but to fish their own size!
Trust me, I found that one out the hard way. Once my catfish reached about 3", I had about 12 different Danios one night, the next day I only had 2. You should of seen the catfish stomachs, they where dragging on the gravel. Now I really don't know that much infomration on Sun catfish. They might not do something like what mine did. But I think there might be a chance that they could.