Too bad House of Mouse isn't being produced anymore.... that would've been the perfect platform for Oswald to appear. After all, this is the show that returned forgotten characters like Horace Horsecollar and Clarabelle Cow -- and even had several B&W era Merry Melodies characters appear in the audience at times!
As for fitting in with the Fab Five -- probably fit in as much as Daisy does (she's the reason why I normally just call it the Fab Five + 1).
Just remember: The one thing that the Fab Five + 1 and Oswald has in common: Pete. Pete is the oldest, still employed, Disney villain. He's been a villain since the days of Alice.
BTW: do a search for Oswald the Lucky Rabbit at Disney.com -- they've really upped the content lately of Oswald stuff, mostly in the Family Museum sections.