I have poked about in the same lines you have, and have just a little information. You may know better than I, but there is a Guy, Guyan, Guian (and other spellings) confusion. My records have Guian as a brother of Charles and Thomas. Charles, born about 1732 in Ireland. No further data on Guyan, nor who his parents were. Charles was my ancestor, so I have descendant information on him. They apparently all three setlled in Pennsylvania. I may have generations misaligned as well.
I think the Guyan information I have was gleaned from either county histories, by me or my Grandmother, many years ago, or is from the Book Mahaffey Descendants, by Estelle Kinsports Davis.
What do you know? I would love to compare notes.