Thanks, Anne. I didn't see this response until now.
According to the tombstone at St. Andrew's, Williamstown, our Alexander Grant would have been born in 1733 and son, Donald Alexander (later just Alex) was born in 1755. Those dates may not be accurate. If the dates you've found and the tombstone dates are accurate, they can't be the same people, but given a few years one way or the other, perhaps they are.
About "ruadh," I know it means "redhead," but I have a hunch that it became a designation passed down through the generations to indicate that the person was descended from a specific family line, a way to distinguish that line from, say, the "bain/ ban" line which had a blond progenitor. What I noticed in my husband's line is that the eldest son in each generation seems to have been identified as "Roy." I have no proof for that theory, but I thought it worth investigaing.