You don't need a B12 midpipe if you buy a slip-on that comes with a new midpipe - which is better anyway, since the stock midpipes on both the 1200 & 600 have restrictions the aftermarket ones do not have.
Since all the B6's have cans welded to the midpipe (as do 2004+ B12s), you can't use a bolt-on can unless you get a B12 midpipe. Since a bolt-on uses the stock midpipe, you won't get as much of a power increase from a bolt-on as you would for a slip-on. If you do go this route, either a 1G or 2G B12 midpipe will fit any B6, but the bolt-on's are different for 1G (2000 & older) & 2G (2001-2003) B12's so be sure you get a mid-pipe and can that match each other.
Keep in mind many of the can makers don't list the B6 (Holeshot does though), but an aftermarket mid-pipe & slip-on combo for any year B12 will work on any B6.