Jiminy Cricket being cast as the Ghost of Christmas Past works in two ways: First is the fact that he is the tiniest Disney character by far, fitting with the first spirit usually being smaller than the other two. Second is the fact that this ties into another work by Dickens, The Cricket on the Hearth.
It's even more on the nose then that. As everyone knows, the Walt Disney Jingle is the song "When You Wish Upon A Star", from Pinocchio. A song sung by Jiminy Cricket. How fitting then, for the character who sang the theme song of the second oldest Disney movie to be cast as the Ghost of Christmas Past.
Scrooge lost his companion on Christmas Eve, which probably explains some of his hate for the holiday.
Pete acting as a good guy, intending to scare the daylights out of Ebenezer to make him pay Cratchit/Mickey a sufficient salary to maintain Tiny Tim's health!
Goofy as Jacob Marley makes perfect sense—in life, by robbing the widows and swindling the poor (his words), he done goofed!
Of course Scrooge would tell the charity workers that they would lose their jobs if there were no more poor people. At that point in the story, it probably doesn't occur to him that they are likely doing that job as volunteers, rather than being paid to do it. This ties into his general greed because he can't imagine that there are people who do work voluntarily out of the goodness of their hearts.
