ταξιάρχης: The Roman taxiarchos

[Nathan Ross]

Marinus, a unit commander (ταξιάρχης)

Marinus is mentioned by Zosimus and Zonaras, but is fairly obscure - his name was probably Tiberius Claudius Marinus Pacatianus, known from coinage. Mócsy suggests that he was given a temporary command (as Dux, perhaps?) over the troops in Moesia and Pannonia, although he could have been a senatorial legatus.

Silvanus, one of his unit commanders (ταξιάρχων)

Silvanus was Tribunus Scholae Armaturum in 351, when he deserted Magnentius at the battle of Mursa (Ammianus 15.5.33). He was later Magister Peditum, and imperial usurper.

some of the unit commanders (ταξιάρχων)

Ammianus (20,4,13) has Julian inviting the officers of the Petulantes and other auxilia units to dinner in Paris, where he won them to his cause. The phrase is generic rather than specific: ad convivium proceribus conrogatis, which I would guess means something like 'he invited the chief men to dinner'. Probably tribunes, perhaps campidoctores too?

one of the unit commanders (ταξιάρχων) was wearing a golden torque which held stones set in gold, they took this and fit it to his head... Pentadius, magister of the imperial units (ταξεων)

The first of these was Maurus, draconarius of the Petulantes (Amm, 20,4,18) - he later became comes rei militaris in Thrace. Pentadius was magister officiorum, and commanded the scholae - these would be the 'imperial units' Z mentions.

many of the unit commanders (ταξιάρχων) kept objecting that what the deserters were saying

This incident in Ammianus probably falls into a lacuna. He does describe a 'conference of the general staff' at this time, described as consilio cum primatibus - which is just a 'council of leading men' I think - so we don't know the exact rank of the commanders. Something like Comites re militaris or senior tribunes is most like I'd say.

who is the taxiarchos and how many soldiers did he command?

It seems this word could be used to describe officers of various ranks, from commanders and senior soldiers of the auxilia and scholae to generals, duces and comites.

This post (and the one following) is interesting about the use of ταξίαρχος to mean centurion, and the possible later development of the word.