I’d bet that Peter at least appears in the 50th because:
-it would suck for the new Doctor to completely miss the 50th
-How do we know for sure that Matt is coming back for Christmas if he cut his hair after filming the 50th?
-Rumors say there may be 4 Doctors in it. 10, 11, John Hurt, and 12?
-Moffat lies (remember that time Jenna Louise Coleman was supposed to debut at Christmas?)