As Andrew says, the default rank assignment is completely useless, I for one never adhere to it.
To change required rank for a formation template: Ground Forces window --> Formation Templates tab --> select formation in the window at right --> "Change Rank" button at bottom-right.
To change required rank for an existing formation: Ground Forces window --> Order of Battle tab --> select formation in the tree window at left --> "Change Rank" button at bottom of window, second from right in that row (this one is maybe not as obvious to find).