Well, yeah you can take the skin part of one model and put it on another but itll look bad. Youd have to model the other face on.
You can do it with any image for a skin. but some models have the parts of the image for the face to go different on that part of the model. Some times you'll see something in one image that is in a different place for the image for the same thing on a different model, or perhaps something is in that image that isn't even in the other (fins, eyestalks, etc.) So you have to be careful what image you use to replace the other. For example: Trying to replace the face for kyle with the face for Jan will look wierd b/c the image is assorted to fit on the model differently, not to mention that the shape of Jan's face is very different.
If you have photoshop, you could distrot the image of another character to fit the model. But that would take several hours, and with just one monitor, a very long time.
Not to mention how weird that would look.