Double Wall Stove Pipe

Yes, as long as it is the same size and brand of pipe you can mix the galvanized with the stainless steel.   

The 6DLR-36ORF should only be used to pass through a damper and smoke shelf before connecting to 6" round DuraLiner, which would then be used for the remainder of the chimney reline. Subsequently, there would be no application where the 6DLR-36ORF would be connected to 6" DuraTech. The only application where DuraLiner and DuraTech would be joined would be atop the chimney in a restoration application where a transition anchor plate would be used atop a partially-demolished masonry chimney in an attic space below the roof line, for example. 

No, 48 inches is the largest for a fixed length and telescoping piece would be a 40 to 68 inch section.

Yes. The DVL double wall black stovepipe is available in 36" lengths in both 6" and 8" diameters.

Double walled stovepipe requires a 6 inch clearance to combustibles and cannot be concealed within a wall or ceiling. You must convert to the appropriate Class A chimney system before passing through a combustible wall or ceiling.

Black double wall pipe is not all interchangeable, as certain brands lock into each other in certain ways.

As double wall stovepipe carries a UL listing, it cannot be altered. However, different manufacturers offer an optional damper like the Dura-Vent DVL 6-Inch Double-Wall Stovepipe to Stove Flue Adaptor - For Wood Burning Stoves.