In terms of the ones easiest for them to make accurate, Scooter, Rowlf, Bunsen and Beaker are the simplest shapes with just a few features added on. Heck, Bunsen doesn't even have eyes, just glasses. And Scooter's eyes are part of his glasses. But with all four of these, you don't run into the carved foam problem of Gonzo.
Chef is a foam puppet and much more complicated shape. I think he would cause all kinds of problems. Also, the puppet Chef's hands are real hands, not even gloved hands, so what do you do for hands on a poser?
Finally, if they have a mold for Piggy's head, she can be made out of almost anything, as long as they can flock it... and figure out how to flock it so the flocking doesn't flake off. Her texture is totally based on the flock look, so the material her head is made from isn't as important.
Personally, I would vote for Rowlf next, then Scooter.