As of now, our current plan has the player making two choices when they train a unit...
1. Unit type: Which unit design do I want to train (Common Footsoldiers, Advanced Bowmen, etc).
2. Grouping Type: How large of a squad do I want? A 'Squad' only requires 10 population units, but a 'Company' of 40 troops will last even longer on the battlefield, with a 'Legion' (300 troops) putting up an even better fight in open war.
Most 'Grouping Types' will require a Logisitical understanding to be met (through Technologies or base racial ability), with some of the larger groups unable to join in dungeon exploration (I would imagine only 'Squads' could realistically traverse most dungeons).
Also, in this system, you couldn't mix-and-match unit designs within the group. That's not to say an Army can't have a variety of bowmen, calvary, and knights, but they would be seperate groups traveling as one body (think Fleets from GC2).
Again, all subject to change, but that's where our our current path takes us. 