My suggestion relates only to March Formation because march orders do not give discretion to vehicle commanders in regards to march order, speed or route.
For the sake of speed of advance, Soviet doctrine was to advance in March Formation until contact was imminent. All Formations would be a mix of infantry, armor, sp artillery and AAA/SAM, etc. Each with different speed capabilities.
As it is now, to model a single company sized inf/armor/spa Forward Security Element in March Formation requires an inordinate amount of micro-management (i.e. the player has to continuously act as multiple NCO's and MP's) to maintain march discipline. Multiple FSE's or battalion sized formations? Way too much work.
As for stacking limits, I don't think they are necessary, enemy artillery will enforce stacking discipline. But if they are, here are a few suggestions:
1. Dismounted personnel should never be an obstruction to vehicle movement.
2. Grid width is several times bigger than vehicle width. A single stationary vehicle is never an obstruction to movement. Noted that this may be unrealistic in some urban old city locations, but overall less unrealistic than current situation.
3. Multiple stationary vehicles in a grid square could impose increasing movement penalties on transiting vehicles representing the burden of having to drive aound the stationary vehicles.