A few thoughts on ground commanders.
1) I am going to have the same options for ground commander ranks as for naval commanders. It will no longer be restricted to just four ranks. This is because you can easily create smaller units than battalions, so you may want more junior officers than Colonels, plus larger overall formations may be possible
2) I will base the default rank requirement on the size of a formation, but allow the player to change that (as he can with ship classes).
3) The physical HQ units (a Unit Class with an HQ component which is part of a formation) will be restricted in the size of formations they can control but there will no restriction on commanders (so you can put a major in charge of a brigade if you wish)
4) However, for a given formation to receive any benefits from the superior formation, that superior formation will need a higher ranked commander.
5) In addition, superior command benefits will be passed from commander to commander, not formation to formation. A formation without a commander will not receive benefits from higher echelon commanders. This should create a natural hierarchy without any up front requirements.
I'll also add some lower level HQ components, so you can create battalion or company HQ units within smaller formations. An HQ unit will be needed if a formation wishes to have subordinate units. For the larger HQs components (Brigade, Division, Corps, etc.), I envision them being a unit that is part of a formation with support elements, such as artillery or AA, with subordinate front line units attached, but in the end it will be to the player how to organise his military (as it is with the naval portion of Aurora).
This might change a little if I ran into issues with implementation, but I wanted to outline the principles.