Alexander MacDonald's army service should certainly be recorded in WO97 at TNA, the National Archives at Kew just south of the River Thames in London. I suspect success in tracking down someone whose name will be common but perhaps spelled any which way may require someone to visit in person. That could either be a family member such as your Clan Donald relative if he lives within reach or, failing that, a paid researcher. Before going to those lengths, however, one or two of the internet data providers such as Ancestry and perhaps Find My Past have transcribed the Military records. That might be the best place to start.
If Angus stayed beyond 1828 only long enough to complete an apprenticeship, it is doubtful he will show up anywhere. I do not think there were the sort of formal written apprenticeship contracts that existed elsewhere (I have never come across one relating to the islands), he would not have had a lease of a croft holding, it is doubtful he would have been in a position to support a wife and so would not have appeared in the marriage or baptismal records and Angus MacDonald was at the very top of the list of names repeated across every township. I have had a look at the Flodda location from which the family seems to have emigrated and there is no stray Angus to be seen there at the 1841 Census or in any of the earlier marriage and birth records starting in 1829.
Alexander is much the better prospect, I think. Best wishes, Angus
