No, I was reading up on it this morning, and basically everyone said, no matter what size the fish the electo cat will kill it, either to eat or they see it as a threat.
One guy was saying that he has a 3.5" electo cat in with 10" pacu fish, and the pacu always stay on the opposite end of the tank as the electo cat, that they seem terrified of him.
But then someone said their synos will actually snuggle up to the electo cat in their cave.
I'm guessing with synos the electo cats give up once they realize they can't kill them.
Just google "electric catfish tankmates" and it's one of the first links.