Having compared all of those census returns and noting that Donald's eldest son was called Alexander I dug around on ScotlandsPeople and found -
MCMILLAN ARCHIBALD/ALEXANDER MCMILLAN/M/27/11/1793/Killean and Kilchenzie
MCMILLAN DONALD/ALEXANDER MCMILLAN/M/03/11/1799/Killean and Kilchenzie
So perhaps no connection there.
My theory is, and it may be a stretch. That Hugh McMillan married Elizabeth/Betsey McMillan and they had issue - Mary 1779-1856, John 1784, Margaret 1786, Iver 1789, Catherine 1791, Florence 1796. Then, Elizabeth dies, and Hugh marries Mary McMillan on 3rd Apr 1800 and had issue - Archibald 1801-1858, Janet 1802-1881, Elizabeth 1804, Hugh 1807-1872, & John 1812.