Again... we must remember overall character dynamic. Sure, the Muppet show was able to get away with hundreds of characters... but I don't know if the same thing would work for a cartoon series where almost everyone has to be in every episode. Perhaps if they pulled a Bunsen and Beaker and made some characters just visit from time to time there would be a better flow.
Plus, I really think that certain characters should have a variation of age, much like Janice had. I could see Dr. Teeth as a kid no older than 7, looking after the kids like his brothers and sisters... I could see (and I mentioned this before) a Teenage Sam who acts older than he thinks he is babysitting and bossing the kids around. And I could see Pepe being slightly younger than the rest, but acting like he's more mature romantically than the others.
Chef I don't see as a kid. 20's or so, maybe... younger than Uncle Statler and Waldorf definitely. Maybe he can be a pizza delivery boy or something. I could see that.