I actually think we should estimate the ammount of hampering each unit gets according to the kind of unit: pikement/halberdiers needed too much time to move, while archers were always pretty agile on the field...

Furthermore, units don't always move coherently: most of the time they keep loose formation for ease of movement.
Earlier I made a statement that what might work best is if each army is assumed to have spent most of the battle time in the final stages of the strategic adjustment. Then the battle being played out represents the shorter time just before negagement. At this point we're not talking about marching formation, we're taling about battle formation. So units should be moving on the field as if they are about to engage or fire missiles.

While pikemen are probably the densest formations, they're also the ones most likely to fight in square formations (historically, this was one of the things the Swiss pikemen pioneered as I recall). Duane pointed out in an earlier post that one of the great advantages of a square (box) formation is its ease of turning by "about-face."

Rather than getting too specific about different formations, though, I think it's reasonable to keep pikes and infantry at the same overall level of mobility.

Irregulars, mentioned in my previous post, are the one unit type that tends to be in loose formation by default. This is why infantry get a melee bonus against them. What is sad is that irregulars cost the same as infantry and archers but are generally inferior...I have a few ideas to even that out, mainly adding a point of mobility to irregulars (1 step faster than comparable infantry), and giving them +2 defense vs. missile fire (one of the main advantages of a loose formation is less hits from mass missile fire).

Archers, hmmm...I debated whether they should have more speed than comparably armored infantry. If they typically move in loose/irregular formation, however, then they should suffer the same disadvantage vs. infantry that irregulars do. What I think is better is to leave them alone: when close to engagement, archers will spread out in long, thin lines to maximize volleys of fire. A 200-man company will probably be about 4-5 men deep and 40-50 wide...which is pretty slow to maneuver, and especially turn, in quick time. So again, I'd say the pros and cons roughly even out, giving them speed comparable to infantry.

If we're working out a battlesystem where units aren't given formation choices, we have to abstract and generalize those things without creating too much extra complexity. It's a tricky balance to achieve, of course, since any simplifying also kills pertinent details of a truer simulation.