The fact that some of you are saying 'why are his characters in the same scenes together?" Well, it's a nice little audio trick called Looping. Gonzo and Beauregard can be in the same scene, and with just a quick recording and talking to ones self, you can have the two characters interact.
And the Rizzo thing? I think it's indeed because Steve has taken over alot of new characters. Rizzo, Lips and Fufu where his Muppet Show characters, and even then, Lips never spoke. Wembley, Rizzo, Bean and Mew almost had the exact same voice. Steve even said himself he barely any difference in his early voicing. Marlon, Jake, Poodlepants, and Andy where all new voices to him, and then he took over Beaker, Kermit, and then, later, Statler. Then, with the new movie, Link and Newsie joined the Whitmire ranks. So that's almost 20 characters. Rizzo's bound to slowly be phased back into the background.
Samething's happening to Pepe, I beleive. Rowlf, Dr. Teeth, and Bobo are all bigger than Pepe now.