Histwar is a high-level simulation. You give orders to the Corps Commanders who in turn formulate more detailed orders to Divisional Commanders. The computer handles everything apart from those orders at the top level (although you can give direct orders at lower levels, down to individual regiments, if you wish). So if you order a Corps to move toward the enemy, the computer will assess the best formation and will move horse, artillery and infantry in accordance with the doctrine for that country (which you can tweak a little using the doctrine editor programme). If you want to move all the horse to one flank you can, but this may confuse the AI.
JMM has always said that the AI works best when you give orders as Army Commander only and do not then try to interfere too much and do not keep changing the orders to the Corps.
We are waiting for more advanced versions of the AI in Histwar Napoleon with the ability to command at Corps / Divisional levels letting the AI manage the overall strategy - there has been a lot of discussion about this feature over the years on this forum.