In the Gamespot Forum, there is a question about how long this game is, how many things were left dangling, and how it may have been truncated because they ran out of time before the holiday season, or ran out of money, or were in the process of moving.
AS FOR THE PETES, it was a phony cliff-hanger. I redeemed all the Petes in Epic Mickey: BIG BAD PETE was not grouchy anymore, but was handing out fireworks. SMALL PETE was in gremlin village, working on his boat with gremlins. PETE-TRONIC was on a rocket ride with gremlins. PETE PAN was his happy self, because I had rescued his sprite.
So, where did it come from, that suddenly there is something that wasn't even hinted at during Epic Mickey 1 and Epic Mickey 2? It breaks all literary rules, and is despicable. It was not planned, and Disney just threw it in hap-hazzardly at the end.
TO ANSWER YOUR QUESTION: No, you cannot find the Petes until you buy Epic Mickey 3 -- and deal with more technical glitches, bad programming, and inadequate story line. - - - - After you see the credits, you can continue doing quests and challenges, but most of them are lost causes and busywork -- like thinning out buildings and jackhammering the streets to find "Hidden Heroes". I play Hidden Mickeys at Disney Anaheim, and it is not like that. Look up "Hidden Mickeys" on the internet, and you will see fun, not being jerked around by mean-spirited designers.