One more bonus point that would go a long way towards distinguishing one line infantry unit from another. Each freshly-raised regiment should have a few(I´m thinking three is a good number) "slots" where you can purchase attachments for the unit(when researched with the appropriate tech of course). A few ideas for attachments:
1. Experienced sergeants
The unit may be green, but it's NCO's are old hands at war, and their experience allows for greater staying power in combat as they keep the rookies in line, and more rapid accumulation of experience as the NCO's impart their wisdom.
2. Integrated artillery
The unit has a cannon permanently assigned to it, crewed by men from the unit. The cannon slows them down on the advance(unless detached as the infantry goes forward) as the cannon has to be pushed by hand across the battlefield. Idea for formations intended for defensive duties and when you need just a bit of extra firepower(such as in colonial expeditionary forces?)
3. Integrated marksmen
A small unit of riflemen that moves and fights with the main unit, can be sent forward a bit or placed at the flanks according to taste. 10-15 men perhaps?
4. Integrated grenadiers
The forerunner to the fielding of whole regiments of grenadiers. Good for both defense and offense.
Such "tricked-out" regiments would of course be more expensive to upkeep, but allow for the creation of specialist formation for special missions.