My friend and I have been putting far too much thought into this concept.
EBENEZER MEOWTH - Meowth is a prick so he has to be Scrooge.
PIKACHU CRATCHIT - Meowth's faithful employee, of course.
TINY PICHU - SPOILERS: HE DIES.
WOBBUFFET MARLEY - WOOOOOOOOOOOOOBBUFETT!
GHOST OF CHRISTMAS PAST - Mew is such an angelic Pokemon and Past is usually the most innocent looking of the spirits. So Mew would be perfect, no?
GHOST OF CHRISTMAS PRESENT - SLOWKING? BUT SNORLAX IS BIG AND FAT WHY NOT MAKE IT HIM? Yeah, but Present needs to be someone who can talk just so he can say "I SEE A POKEBALL WITHOUT AN OWNER" when Scrooge asks about Tiny Tim's health. And Slowking seems like a nice fit.
GHOST OF CHRISTMAS YET TO COME - Yes. Ghost. The ghost you see before you get the Sliph Scope. Why? Because it's it perfect and you know it is. Future is always the most mysterious and disturbing of the spirits and Ghost fits that role perfectly.
FRED - Scrooge's nephew. Why? Because Charmander just seems like he's friendly enough to keep inviting Uncle Meowth to Christmas dinner even if Meowth is a raging douche. How a Charmander and Meowth could be nephew and uncle? IT'S A MYSTERY!
WOOPER FEZZIWIG - Because I could picture him dancing during the Fezziwig party scene. And because Wooper is awesome.
BELLE - Scrooge's girlfriend from the past. Come on, it would have to be Meowzy from "Go West Young Meowth". It's perfect.
FAN - Scrooge's sister. Jigglypuff was the first to come to mind.
THE PORTLY GENTLEMEN - You know those two guys that come to try to get a donation from Scrooge? Just picture these two with top hats and monocles and suddenly it makes all the sense in the world.
PIPLUP CRATCHIT - Pikachu's wife. Just so we don't have two Pikachus.
If I had the patience or time I would totally make this into an entire flash cartoon.