I believe they are the same length 17.5 roughly. The thing you will need to know is whether or not the rese will work. The two shocks are different in how Therese is mounted, rs is with hose and clamps, FX is casted into the cap. This may or may not cause interference with the track when fully compressed.
Honestly, I would get a Fox Zero Pro and add the rese now or later, depending on funds. They are relatively inexpensive, rebuild kits are less than twenty bucks and just about anyone that works on shocks can work on Fox.
As for rebound, just have it valved correctly and don't worry about expensive parts that you rarely touch. Rebound adjustment for a trail rider is useless in my eyes, that's assuming quality valving had been installed to begin with. Sure you may have a few instances where the sled bucks you, then you turn up the rebound, the next day it's sacking out so you put it back or at lest close to it. There are too many variables in trail riding that make rebound adjustments pointless (after a set point has been established). Again this is assuming you have a proper valve setup to begin with. Which by the way may take a time or two to get just right. My opinion. I should say that I have a bit of a distaste for the stock shocks and Yamaha's lack of part availability.
