I did my stock, adjustable Showas on a 96 900SS/SP a few years ago. From what I can remember, I made a couple of special tools, but not that special. I cut a slot in a washer to keep the spring compressed at some point. Also, I got a piece of PVC, I think, as a seal installer for the assembly.
A spring compressor would have come in really handy as that part was a little tricky. If I do it again I'll buy one or build one. I've heard of people making a spring compressor out of 2X4s, allthread, and hardware.
I put different springs in and RaceTech Gold Valves and I really like the change, though probably mostly due to the springs.