Yes, be careful with some other John / Margaret MacKenzie couples that pop up in Gairloch. There are at least these other two marriages:
27 JUL 1810
31 JAN 1812
Looking at all the OPR's for births in Gairloch between 1800-1805, the only record for a John whose parents were John & Margaret is the one from Melvaig baptised 5 MAY 1805. I'm sending you a PM regarding this.
If that is your John, then he also had a sister called Margaret, baptised 1 JUN 1810 (not to be confused with another Margaret, same names for parents, baptised 17 MAY 1810). She married John Chisholm (probably 14 FEB 1831) and died 30 JUN 1895 in Melvaig.
Cheers,
Kiko