The acrylic is already "scratched". We buy 1" clear acrylic, and sand it with a random orbit sander using 60-80 grit pads. I suppose it will scratch like any other surface such as wood, if you drag a metal edge across it. The good thing is you can easily sand out any scratch. In an odd reverse, the main traffic areas actually get smoother over time, not scratched.

As far as installing over an existing stair, it seems odd, as the main benefit is the light transmission. If you were to do this you would want to space it off the surface below by about 1/2" so it does not translate the surface below. If you did this you might not need 1", if it is supported correctly. If not supported well it can sag over time.

The material is standard acrylic, you can buy cast acrylic or extruded acrylic, in the end they look the same. We but form a big wholesale supplier, here in Nor Cal, it is called Sabic, which I believe is a national co. Avoid the plastic retails like Tap, they are twice the cost.

what about a cat's paw?