Urchin VS Acrylic

I have a 46 gallon Acrylic tank and I've been wanting an Urchin to do a little GHA mowing, but I've been told they can scratch acrylic tanks, and this would be a no no. I know the spines on a short spine and a tuxedo should be more than short enough but will they still scratch it?

Also my fiance has been asking if I can find a Purple Short Spine Pincushion Urchin (Pseudoboletia sp.) and I know foster and smith offers them but how rare is this species?

 

i have acrylic and have white with red short spine urchin, also i had 2 of the purple urchins you speak of, they eat star fish, it ate one blue linckia i had for over a year, than when i added another the one attacked that star fish, i also seen it go on the sand sifter star, which wend down into the sand and saved its self, the one white with red urchin i have never seen go after the star fish. its also a short spine urchin.
i can tell you with both times i seen the purple eating the star fish, it was always just one of the purple, never 2 at once. so i don't know if just that one was a star fish killer or they both were. but i did remove them both from my tank and plan to add another linckia star fish.

oh and i have never noticed any scratches on the tank from the urchins. i would think long spines urchins would be one to worry about