The Grizzlies with the 12.5" barrel are identical to the 870. All parts are interchangeable (except the mag tube cap, which is threaded ever so slightly different). The barrel will fit on any 870 with a standard 4 shot mag tube, and the grizzly will accept any 870 barrel (unless it's from an 870 with a permanently extended mag tube, like some of the tactical versions). The Grizzlies with the 8.5" barrel will not accept any other barrel other than the 8.5", since the mag tube is too short.
I own several Remington 870s and a Grizzly, and the parts are currently pretty mixed up between the two. If you want a short barrel shotgun, buy the 12.5". 4+1 ammo capacity, and swappable barrels. Plus you can buy aftermarket fore-ends and be sure that they'll fit.