Don't call it necroing, I was STILL looking for input over this, anyways
I intended the DR to stack. Impenetrable Hide and 19 levels of Barbarian, for example, would net you DR 7/-. Mechanics-wise, it is an unnamed bonus, so it should stack with everything
I guess it ISN'T very clear though. It just says you gain DR 2/-, not that you gain a bonus to DR or whatever. Frankly, I don't know HOW it should be written. Maybe if I add this line it would make it clearer?
Impenetrable Hide [Mirran]
The adamantine in your skin hardens even further, to the point of letting you ignore some damage.
Prerequisites: Con 17, Armored Skin, base Fortitude save bonus +7.
Benefits: You gain a damage reduction score of 2/—.
Special: If any other source already gives you this kind of damage reduction, increase that damage reduction value by 2 instead.