Yo, I did a bit of testing and I think the Skin 6 is impossible to gain on new children as a heridary trait. I made a new save with all characters sharing mostly the same features, and bred them for 80 days. Not one ended up having Skin 6. Interestingly though, all the children ended up having Skin 4 with no variation.
Tried to do the same in my main save, where I only had one character leftover from the start (with the Immortality Gem) with Skin 6 to no avail over a few generations. And though I'd try testing this.