If it were me I would leave thelate Roman unit sizes the same as their predecesers, why? Well That way three battlefield units always represent a Cohort. Whether you fight in three thin lines with four cohorts on line or in two deeper, staggereed, formations is up to the player.

As to the issue of officers in phalanx, well short of using shieldwall in 1.6 I think we can live with it. Its really not something to worry about unless your line is all ragedy to begin with.