I found set skin is only useful when applying a particular skin to the base of the model as a whole so I could choose any one of my skins to be the overall skin rather than the default top one, havnt experimented any further with it though.

What you need is to have a number fo skins for the animation and choose them via script to show the animated skin.

If you added the head as a seperate model onto the body then that would make the skins smaller and easier to handle as only the head skin will be changing and not the entire models skin.