Importance of "H.E." rim designation on MTB to tire fitment?
I suspect "H/E" means "Hook-edged". This would apply to almost every MTB rim.

I have two bikes with slicks on MTB rims. I currently use Schwalbe Kojak 26 x 1.35" on one, and Schwalbe Marathon Racer 26 x 1.5" on the other.