I seem to remember that Sith Purebloods have their own slave caste, in which case a pureblood SI doesn't contradict anything lore-wise. Saying that, though, I made my SI a pureblood without knowing anything about the back story. I mostly did that, however, because I wanted a Cyborg for the SW story. I'm also rping that he's an ls-person that's been humbled by his life in slavery. Kinda unusual, considering his species.