I always figured that it depended on deployment. In space, US Navy or British Fleet Air Arm ranks would best apply, but on the ground MS seem analogues to a mix between armor and infantry, the latter especially in their tactics. They wouldn't really count as aircraft because - at least during the One Year War - all but a handful of trial-production prototypes are incapable of sustained flight.
And that's fine, I'll go ahead an' change 'im.