You forgot Sir John Elley who rose from the ranks to become the Lieutenant Colonel of the Royal Horse Guards, knighted and a General Officer. One of AW's trusted Staff Officers.
Henry Hardinge was involved when Harry Smith was denied a peerage because of his humble background and lack of resources to support a title, as well as passing him over for the chief command in the Crimea in succession to Lord Raglan, when Henry was the CinC of the British Army.
Henry and Hugh Gough both received peerages [and money from the HEIC] after the 1st Sikh War. All Harry received was a baronetcy.
Wellington gave Harry Smith high praise for the Battle of the Aliwal 1846, dubbing it the battle without an error.
It was the fashion to have one's relatives, officers from one's regiment and those suggested by others on your staff, titled or otherwise.
Being the son of an Earl, brother to a Marquess, serving Earls and Dukes as Chief Secretary of Ireland and friend to sons of peers of the realm, it would hardly be surprising that AW chose from this circle. Given his history with David Baird, would he have turned down the offer of Baird's own former ADC, Alexander Gordon because he was the brother of the Earl of Aberdeen? Gordon was also ADC to the Duke of York, CinC of the British Army.