The Sedran Colonial Guard is probably a component command of whatever successor the UNSC established for the Colonial Military Administration. It's likely not anywhere near as independent as the former CMA was in consideration to the UNSC, but we know that it does have a modicum of autonomy from their command structure given Colonel Aiken's reluctance to have ONI assets on their territory and with their procurement of arms and vehicles.

Putting all that aside, however, the SCG would still be considered an allied fighting force alongside the the UNSC and CMA. Within multi-national alliances like the former SHAEF and Warsaw Pact, and the modern NATO, ISAF, or the United Nations Command, all nations follow a general rating structure for both officers and enlisted persons to cement authority across joint forces. For example, a First Lieutenant in the U.S. Marine Corps and a Lieutenant in the British Army are equal to each other, but are both subordinate to a Flight Lieutenant in the Royal Australian Air Force, a rank equal to Captain in both their services.