The warning about plugging a IIgs monitor into a IIc exists because the IIc *does* have a 15 pin "video expansion port" on it that you could confuse with a "normal" monitor port and, yes, plugging into that may well let the magic smoke out of either your monitor or your IIgs. The port on the IIc brings out TTL-level signals from several places in the video generation circuitry and isn't suitable for connecting any sort of "normal" monitor directly. The most famous thing sold to plug into it was the IIc's matching monochrome LCD panel, but I also believe that several RGB adaptors existed. Again, though, any such adaptor contains active circuitry.
Obviously terrible things would also happen if you were to plug your IIc LCD panel into a IIgs, but the design of the case would make you have to work really hard to pull that off.