Rank Structure of the Late Roman Legions

[Robert Vermaat]

The Comitatensis were orgainsed around the concept of a legion or auxilia for infantry regiments, and vexillationes for cavalry regiments. Numerii seems to something of a catch-all phrase to describe any body of soldiers, at least from what I can tell.

That's ComitatensEs.
Like I said, vexillationes could also have been smaller parts of infantry formations.
Comitatenses were the units with the most status, most of them belonging to field armies.
Limitanei were mostly border troops (NOT militia or farmer-soldiers) whose job it was to hold out until the field army arrived, or defeat raids and keep the peace on the border.

So it would appear that the ratio of cavalry to infantry seems to have been 1:2.

Or less.

Elton goes on to claim that these regiments would often operate in pairs, forming a battalion under the command of a Comes. What he doesn't say is if it were possible for a Count to command a larger force, as in the case of the Count of the Saxon Shore.

Which is indeed the case.

Barbarian foederati are mentioned, but Elton doesn't seem to know just how large foederati attachments to Roman brigades would have been.

They would not, depending on numbers they mostly operated in units separate from the regular army, often under their own leaders.

As I understand it, the upper echelons of the Roman military worked as thus;
Migister militum commanded Duces within his diocese. The Duces in turn commanded Comes in their individual provinces.

No, that's incorrect.
The magister (utrisque) militum was the chief commander of the forces,
Earlier, the men below hiom would be the
magister peditum (general of the infffantry) and the
magister equitum (general of the cavalry), but that difference seems to have vanished around the late 4th century.
below them came regional commanders, the
magister militum per (diocese or province X or Y).
below them came the rional commanders, Comites and Duces.
A Comes always commanded a field army and was hence higher in status than a Dux.
A Comes, however could, like a Dux, also command a regional army group, indeed like The Count of the saxon Shore or the Duke of Britain (who commanded the region of Hadrian's Wall).

Here is a good list of the ranks below them: http://www.geocities.com/thebatavi/lraranks.htm