Does anyone know if you can use a non-super mag slug barrel on the 11-87 Super Mag if you use only 3" shells?

I checked the Remington site and its suggested you not mix barrels, but I am not sure why. Does anyone know?

The reason I ask is that a local chain store has an "exclusive" version of the 12ga 11-87 Super Mag in camo with the front True-Glo site on sale for $579. Also, my father in law has a brand new 21" cantilevered slug barrel with a nice Nikon scope he said I could have.

Any help is appreciated before I make the plunge!

Thanks,
Brian