[Mobhack]

There is no way to reorder your core other than selling them off and repurchasing new units in your desired order and losing any experience.

If you want to add units to an existing platoon, use the cross-attachment routines, though you cannot cross-attach a commander of course. So you may be left with a stub formation that you need to delete. If the "0" unit goes squiffy after cross-attaching units to the formation then if you have the full game you can use the "repair unit leader" function to fix that.