The Aberdeenshire Chalmers

I have John Chalmers b.c1810 at Monymusk (?) was a Lance Sargeant with the 21st Fusilier Guards. Who was his father ? I found a John Chalmers who died of diarhea at Scutari in Jan. 1855 but he was a corporal with the Grenadier Guards. Are they completely different units? And is a corporal under a lance sargeant ?? Maybe he was demoted ???