Zodiac and Late Roman Army Organisation

[Flavivs Aetivs]

[..] we would normally wait for some scrap of evidence to suggest that a legion could be divided up amongst the tribunes, before proposing it as a general rule. Or again, some evidence that legions were reduced to 1,000 men, before proposing that a tribune might be an appropriate commander for such a size of unit.

Did Mommsen propose it as a general rule? I haven't read his actual words, I only know that he was the first to propose that this was perhaps how the classsical large legion could have been broken down into smaller units.

Aren't the ND legions commanded by praefecti?

The praefectus was the officer in command of old-style legions (praefectus legionis) and of old-style alae (praefectus alae), although these were only to be found in the West, notably on the Danube and in Britain. Praefecti could also command several units together, as seen with the praefectus legionis quartaedecimae geminae militum liburnariorum cohortis quintae partis superior, Carnunto, who commanded the fourteenth legion as well as a part of the Danube fleet plus the fifth cohort, from his command post at Carnuntum.

The tribunus was the commanding officer of a new-style unit, which could be a regiment of auxilia palatina or a numerus or anything in between. Tribuni of the scholae were commanded by the magister officiorum, but tribuni also commanded cavalry vexillationes, new-style auxilia regiments as well as the new-style legions of the field army, but also the old-style cohorts of the limitanei. By the mid-fifth century a tribunus might also be styled a comes, under the debasement of Roman military titles. By the sixth century a papyrus describes an old-style cohort commanded by a tribunus, eight senior officers including the adiutor (regimental clerk), the primicerius, six ordinarii and six others, probably the centuriones.
A so-called tribunus vacans was an officer temporarily without unit serving as a staff officer. These tribuni vacantes could also serve on special duties – when Ammianus was on a misssion from Ursicinus to relieve the magister peditum Silvanus of his command (read “arrest him”), he and his nine fellow domestici were accompanied by several tribuni vacantes. And in Egypt, a tribunus civitatis might combine military and civilian duties, acting like a governor. Tribuni could also be in charge of barbarian groups, as the example of the Tribunus gentis Marcomannorum shows. We know of one Agilo who was a tribunus stabuli in 357. These men (later comes stabuli) were responsible for gathering levies of horses for the army. A tribunus probably received eight annonae (plus four capitus if cavalry).

There was also another term, can't remember though but I think it started with a P and it meant command over the limitanei garrisons.

Also, you are right. Palatina were named Seniores and Iuniores USUALLY, but not always. The Cornuti Seniores, for example, were a high-grade infantry unit that shifted stations but were usually seen in Gaul under Julian through Aetius. But the Cornuti Iuniores I think were a cavalry Unit in Italy or Africa, not sure. (I don't have access to my ND link right now.)

Also, about the Magister Peditum and Equitum - what sources really do we have for those ranks, because in most cases it seems it was Just Magister Militum. (As in MAgister Militum per Gallias, Tracium, Itallias, Hispaniensis, etc.)