I don't have any PORTERS so you may have me confused with someone else. So far as my ALEXANDERS. . . . the furtherst back that I can document is James ALEXANDER b. 8 Apr 1749 in Pennsylvania - although I don't know where in Pennsylvania. The date of his birth was recorded in his family Bible which is now in the archives of The Genealogical Society of Iredell County, North Carolina. The place of his birth was mentioned in his Rev War pension application.
I have no information on his parents and so therefore thinking about where in the British Isles they came from or when they arrived in America would be pure speculation.
James ALEXANDER and his wife, Elizabeth ROSS, did not seem to use the standard Presbyterian Scots naming pattern. At least, if they did, I can't seem to see the pattern. They did not name any of their children after themselves: no Elizabeth and no James. They used surnames as middle names (WILKES, THOMSON, CUNNINGHAM,SHARPE) but I can find no connection to any families of those surnames.
So - at this point - I can't see any connection between your families and mine.