Here's my secondary reaction to this concept, being how I might implement it usefully in the game:
Personally, I find the games current implementation of armies unrealistic and felt that it needed revision (easier to build armies, for one thing).
So I'd like to retain the single-figure units as representative of irregular figures (tribesmen, warbands, skirmishing horsemen, guerillas, et al), and use grouped units to represent the following:
1. Organized formations (ie, organized armies dedicated to pitched battle, such as hoplites, legionaries, and musket infantry).
2. Numerical degree, with the attendant multiplication of cost (including population cost in my personal mod).
So, an army/grouped unit is any unit with the army flag, and these army units can built without leaders once a particular small wonder is built (based on an ancient tech like Military Bureaucracy or Pitched Battle).
Non-army, irregulars can still be convened into larger forces by virtue of the old fashioned Leader unit forging them into an army.
In this concept, only units that employ tactical formations would be grouped. All others, being irregulars or otherwise using retreat tactics by definition shouldnt be coalesced into armies (and therefore not allowed to group their attributes the mark of a formation). Im not sure how this would apply to most mechanized units. Off hand, Id say that bombers with fight escorts would apply as an army formation. I dont know enough about tank fighting tactics. The distinction between organized cavalry and light, skirmishing horseman would be clear.
As an aside, perhaps modern infantry should be in single file formation.