What the need most of all is for the posers to be able to interact with one another. Animal can do that with Kermit in a limited way. There is only so much that Animal and Kermit can do together.
Gonzo and Kermit will make great companions. We can set Gonzo up in a variety of situatios, with Kermit reacting. Kermit and Gonzo have never been best friends, but they are closer than Kermit and Animal.
Fozzie will be simply great to have with Kermit, because Fozzie is always grabbing Kerm, hding behind him, whispering to him, etc. So there will be dones of great poses for the two.
That's the reason they should do Piggy or Rizzo. Rizzo can interact with Gonzo, and Piggy with Kermie.
I'm just not sure who Beaker or Pepe would be able to pose with?