Maybe you could have a torso with X number of holes in the neck area and a pack of "plugs" and heads, so that people could, in effect, choose themselves how many heads thwere were going to be. Personally, I wouldn't want 12 heads, between 5 and 8 would be better for my taste.
Re the head options, yes, they should be 'uniform' - I would just have two or three different head packs to choose from, a bit like the Giant has 3 head variants. As the model would be so large (and premium priced) this may be an economically viable option?
Just a suggestion.