Read Thread: bought Pfeil's L 12/1
Kirbert has written up a pretty good tutorial.
It's here, if anyone wants to look at it:
Either one will probably take some getting used to, as these lino cutters are designed to be held in the palm for applying enough force to cut lino or wood. Carving rubber doesn't require as much force, so some of us mount Speedball nibs in Speedball pen holders and hold them like a pen when carving.
I don't think the Flexcut moving will be a problem when cutting rubber. It might be an issue cutting wood, but it's such a quality product that I can't imagine it's a problem. It's more likely a feature!
IIRC, the Flexcut nib can be separated from its wooden handle -- which may be a plus for rubber carving. You could either devise a lighter handle or possibly carve with no handle at all.
If you need your new tool modified, I'm available. I have modified a couple of these things, but not many; I can appreciate that someone paying that kinda cash for a new tool isn't anxious to send it off to me to hack it up.