Well, Rocco was under a cloud after the catastrophic failure of his security regime, and, I would imagine, Michael left Neri - whose loyalty was beyond doubt - to keep an eye on his family. And Rocco.

As for Bussetta, I have no idea. He can't have been an old Family associate because Fredo had never met him before. As you point out "He comes in out of nowhere after the Tahoe shooting ..."