I think the thing about regiments being increased in size to where they are around 1,000,000 men or more makes sense. Regimental scale can be entirely different in the
40k universe especially when you look at the scale of things. This is alot of people for a single commander to control though, so perhaps regimental advisors get involved more to help with running the regiment as well as the commisariat. From what i understand an
IG regiment is usually led by a colonel then theres a second in command, a major, then companys led by captains, and platoons run by LTs, and squads by sergeants, and their corporal as an
ASL. For particularly large regiments, around 1,000,000 guardsmen, as i said regimental advisors could have a more hands on role. The commisariat also could help

or for example a Lt. colonel leads the regiment a colonel is second in command, majors lead say 4-7 companies, and you have your captains running your companies, etc. Besides they also combine forces, to form battle groups, 9th battlegroup of the Segmentum Obscurus for example, and regiments are organized with only a specific purpose, infantry regiments consist of infantry some chimeras and sentinals, armoured regiments of tanks, artillary of artillary, so no regiments a self sufficiant fighting force in case of rebellion.