It's official: Rick Rubin the legendary producer responsible for everything from pairing Aerosmith with Run-DMC to helping the Dixie Chicks sweep the Grammys last February is joining Columbia Records in an executive partnership with company head Steve Barnett. He will head a roster full of big guns like Beyoncé, Bruce Springsteen, and Bob Dylan, though he's also expected to continue working with outside acts. An internal memo at parent company Sony BMG which claims more than a quarter of the current album market share says Rubin's hire will move Columbia ''to the forefront of artist development while building a new business model.'' How that model will be structured is anyone's guess, as rumors of mass firings, artist reshufflings, and even a total relocation to Sony BMG's West Coast offices sparked instantly. (Rubin is staying in L.A.; Barnett is based in New York.) ''No one knows what's going on,'' confides one staffer. As for the day-to-day business of running the label? It'll likely be a division of labor. Says an insider, ''Rick is all about the art, while Steve is all about the business. It's a yin-yang thing.''