It's odd that folks are willing to take advice from those who've kept discus/goldfish (wtf are they doing putting a plec in with a goldfish in the first place?!) but not from those who've kept plecs. I've had plecs, big and small, and not one has ever latched on to a fish, let alone killed one (god that was a good laugh - thanks for that!).
No wonder there's so many commons in fish shops with folks believing that kind of tripe. I dont discount there may have been the odd problem with a discus, but killing goldfish? I wouldn't take advice from anyone who keeps coldwater and tropical in together, as their problems probably lie with the fact they keep incompatable fish together, and their conditions must therefore be poor.
I doubt a plec could kill a goldfish somehow - and even if ONE did, it's certainly not the norm. Sorry, still laughing at that one...