Hi, With regard to your query, the first thing I would say is that you are talking about two regiments not one. There may have been a 91st Argyll Highlanders 6th battalion but not 6th Foot. My wife's great-grandfather John Darcy, was born in Ireland, joined the 6th Foot Regiment in about 1846 and then transfered to the 26th Foot Regiment in about 1866.
I have been told, by an expert in Victorian Army Regiments, that at that time many soldiers transferred regiments to either stay in a place, e.g. India or to avoid going to a posting such as India. Some transferred many times.
Presumably your relative transferred from the 91st Argyll Highlanders to the 6th Foot. The National Archives should have his army record in Section WO97 under the 6th foot regiment. After 1872 soldiers were listed alphabectically rather than by regiment.
Richard.