You certainly can take off the luggage rack on the Triumph side rail mounts and run just a sissy bar, or interchange the tall and short sissy bar. None of them are one piece with the rails.
When I first bought my Speedmaster, I also bought the tall sissybar. Later on, when I realized I needed the luggage rack to mount a large tail bag on the back of the sissy bar, I bought one off of a member on this site...just the rack, no rails(which you can't do through the dealer). Mounted it on my sissybar with 4 bolts and nuts.