These are considerably thicker than the rulers used for rotary cutting. I have not nicked mine at all. Sadly, I can't say the same for my rulers.
I have used mine on top of a ruler, which was on top of the fabric. It was not nearly as slip-free as when used directly on the fabric. This makes sense, as the pressure created when the balls on the bottom of the Slidelock are engaged is created against the ruler - not against the fabric.
My preferred rulers are Creative Grids - not because of the built in grippers, but because of their markings. So I just put a piece of yardage, folded twice, on my mat. When I put the CG ruler on it, while pressing it down as hard as I can, the whole thing will still move on my mat. When I put the Slide Lock on the fabric and press down without straining, the fabric didn't move until I pressed down and pushed away from my body as hard as I could - and even then, it didn't move easilly. I wish I could video it to show you!
I've tried lots of ruler slip prevention tips, tools and gadgets. My favorite prior to this were the True Grips, but there was still inevitably some wiggle, or variation, when cutting more than 2 or 3 layers. In addition to the strip of acrylic and wood handle, there are balls the length of the Slide Lock that are engaged with light pressure on the handle. They go through holes the length of the Slide Lock that they go through, onto the surface of the fabric itself. It's not a matter of pressing harder to get the ruler to not move, it's that the balls are actually in contact with the fabric. Release the handle, the balls are disengaged, and the Slide Lock slides across the fabric.
As far as using the June Taylor Shape cut with a different cutter - I didn't try that, but had used two different Shape Cuts with an Olfa rotary cutter and didn't like them at all.
Having not heard of the Rule Steady, I looked it up. Currently, the price is about $10 more than the longer Slide Lock. That's before shipping, which I didn't check.