Madill Family Genealogy Forum
I did some research for a friend a few years back, looking for Madills in NY.
I have an 1850 census record for Lisbon, St. Lawrence County, NY, for Abel Madill, 2nd, and wife Catherine, and they had a son listed as Alexander, age 7. Close enough to be your Roy Alexander?
Abel was, I believe, the son of David and Isabella Madill, who lived next door, and came from Ireland. And I think David had a brother named Abel, who also lived in Lisbon, which is in the same county as Ogdensburg.
If you're interested, I can send you a transcription of my census notes.