The question is how large are the units you are considering dealing with, and how much structure is there in them?
Ultra simple setup in a very primitive rebuilt society would likely only use three to four ranks at best, if not just "Leader and Followers", simply because the higher ranking titles are born out of a need for organization.
Something as simple as:
Throw in special titles such as "Runner" for say a sergent who was trusted with carrying orders between units for added flavour.
Personally I think it is just weird when writers try to suggest too complex of a force with too many ranks being used, and then have no actual need for that kind of structure.