, all the 'skills and drills' and 'special equipment' doctrines save for close order drill (affects infantry platoons and conscript platoons), and carapace armour (guard infantry and/or rough riders) state they only affect guard infantry, who are defined as well.
I think the points on sharshooters have been pretty-much made by everyone else. The only question I have is whether or not, when writing up an army list, you assign advisors as part of the squad, or attach them at the beginning of the game. Using the former, it could be possible to 'unlock' sharpshooters for certain squads, though most (if not all) candidates wouldn't benefit.
The Imperial Guard: We have more men than you have bullets!