Custom model or edited from an existing one, it's still wise to build it around the skeleton of the character it's intended to replace, or else you're BOUND to get size/proportion problems when Brawl tries to fit it to the old character's skeleton.
@thanyou, are you sure that those aren't just the trophy models? I couldn't find any confirmation that they weren't.
@Tiberious, I'm sure all they'll need are the #00's for Pikachu and Jiggs. And FBX would be a safe format, since it preserves most things and a free converter for it exists.
But then again, I'm not the one who needs them.
@Rkey, yes you can make any model you want. It works almost EXACTLY like a texture hack (models are actually stored in the .PACs). Have you read the thread at all?