This is an interesting topic indeed. When I first bought my electric eel at 15" I kept him with a 12" common pleco. Once he was past the 21" mark though the pleco became to swim fast and go crazy bumping into the filter intake etc... so I moved him. I believe the pleco could not take the shock from the electic eel. Keep in mind though the tank was bare and my electric eel "liked" my pleco as he would go against the pleco and they would sleep side-by-side. I have seen at the aquarium an electric eel be with all kinds of fish, some are smalller, that tank though is 400g+ and there is lots of hiding places as they try to re-create the tank to make it look "wild". I would think that if you had large fish that were to big to feel the shock then It would be okay.