I've cut down a few aluminum capacitive scales, no problem. The stainless steel versions are reportedly more difficult to cut because the SS work-hardens pretty quickly. In that situation a rotary tool with an abrasive cutoff wheel will do the job.
When cutting, grip the waste part in your vise. You don't want to damage the rule structure on the scale.
BTW, given my experience with the aluminum scales, I can't recommend them. Due to contact problems between the scale and an internal sliding contact inside the sensor body, they seem to be more sensitive to noise. If you are installing them on a machine that has a variable-speed motor and PWM style controller, you could be in for a lot of grief. The stainless rules cost a bit more but (so far) my stainless-steel rules are much better behaved w/regard to noise.
The best solution, if you can afford it, would be to go with magnetic or optical scales. As an amateur machinist I found it hard to justify the cost, but, given all the problems I had to resolve, I would have been ahead of the game had I just bit the bullet and purchased the good stuff -- no futzing with the DROs, just using them to make things. Buy once, cry once really is true.