Formations are separate detachments. Same way an allied detachment is a separate detachment from a combined arms detachment.
Same way the masque detachment is separate from formations, which are their own detachments. A formation acts like a detachment, but instead of its requirements being things like 2 troops and an hq, it spells out individual units.
Your list only contains enough models to use the mature detachment. If you wanted to use a formation as well, you'll need to fulfill its requirements separately and it will only gain the command benefits of it's detachment and no others (and vise versa).
Does that make sense?