I kept a synodontis petricola with my electric cat for nearly a year. Didn't think it was going to work as the e cat had already paralyzed and ate 2 weather loaches that were as long as he was (about 4" at the time). The petricola was introduced less than an inch long, the e cat was about 5", his mouth was as big as that tiny thing and I was certain he'd be gone in a day.
To my surprise, they liked eachother. The petricola seemed able to sense the e cat's charge and would follow him around nearly 24/7. I always knew where the big guy was in his pvc tube because the little guy would be on the outside making figure eights above him. He even wandered in and out of the tube with no trouble right in the e cat's face!
I kept reading that synodontis and e cats can be kept together until they reach about 8" with no further explanation. That is exactly what happened to mine. E cat hit 8-9" and the petricola (who had gotten to 4"+ by then) just disappeared one day. I held out hope for a week, but he was never seen again. They may be immune to the jolt, or maybe it's something else, but I don't put it past these guys to eventually eat anything and everything. Just look at them, the whole front end is a mouth, lol!
The only fish to survive the entire time he's been in there are glass cats (kryptopterus minor). They tend to stay away from the bottom and prefer near the top by the HOB return so they have survived. However, eight of them have also been eaten. One simply liked the bottom, and every time they are badly stressed (when they were taken out of QT and recently after a move) they scatter among the rocks and driftwood for a few days and more get picked off.
I would like to try a pantadon butterfly with the big guy. They stay at the top and I love feeding them, just worried if he got eaten it would hurt the cat. It's about the only thing I wouldn't EXPECT so much to get eaten. Mine has recently started climbing up the sides of the tank to near the top looking for more food after he vacuums it all up and light no longer bothers him at all, used to be he hid from light like a raph cat.. I don't think anything is safe in there.