Why would you use the KSS60 on the FSK rails? The Mafell comes with its' own crosscutting rail that works really well.
You said you have 800, 1.4m and 3m guide rails - which would be the FS rails. The Mafell works on the FS rails, as you mentioned, by removing an insert. It's really not that hard to remove.
The alternative is to invest in a couple of the 1600 bosch rails and the rail connector. You don't need a 3m rail since the Mafell/Bosch rail connector works 100x better than the Festool joiner.
There's also a Flexi version that comes with the smaller KSS40 - it rolls up in a systainer.
I still have a FS1400 with holes for use with my OF routers, but otherwise switched over to Mafell/Bosch rails. Yes, less accessories, but better in other ways like alignment, rail joining, clamping slot near splinter strip, etc.