I think that Shade is not an incarnation of Link because his shield doesn't look like any shields Link used to have (which is a major supporting theory for his relevance)
I don't think that matters. The Stalfos form isn't the Hero's Shade actual incarnation. The golden wolf is. He just transformed into a Stalfos for combat purposes. He also said he accepted life as the hero and references a bloodline between he and Link, so it's pretty heavily evidenced that he's a previous Link. And, honestly, why would some random person seek out Link to teach him skills? That seems very unlikely.