Personally I don't think Plageis is Snoke.
-For starters Plageis is a Muun and Snoke, looks to be a different species. I don't know what he is, but Muun's have more elongated skulls in the forehead/jaw area
-Second, I'm not 100% sure that Snoke is a Sith. Yes he clearly uses that force and the dark side at that, but that alone does not make him Sith. The title "Darth" was missing from both his name and his apprentice, and I would guess it's intentional.
-Third, it just makes no sense to pull an obscure character out of no where, which will ultimately create more questions than answers (what did he do between his "death" and his re-emergence, why not capture Luke and make him your apprentice at the end of ep. 3) as opposed to create a new character and making their back story.