I guess what may be needed is two movement buttons, one as is another where formation is secondary to position. Or as you suggest different troops do it differently, though when I was playing as greeks most of your guys are phalanx and they would all suffer reformation (and congestion) at gates.

It also strikes me that the basic design of the game engine may not allow this modification , it would be nice to know if it was possible to change the reformation point from CA.