I would go light fast rolling for the rear. On the trail boss. The new light ones have a slash guard on them so are still pretty sturdy.
No, no, no. The point of WTB’s light grip and tough fast options are you can run the light one up front for grip and the tough fast rolling one out back, especially on a hardtail if you ride fast tech stuff. Their tough fast are dual ply, which if you do ride a HT like a FS on fast trails, is the minimum tyre spec for the rear to avoid issues. Even depending on your weight, you shouldn’t need more than 20psi out back with this setup.