Cohort commander?

[Frank]

Well... as I admitted I am no expert on Roman military details, so I really do not know whether a praetorian legion counts as normal

Depends on what you call normal. For early-/mid-empire NO. The praetorians were not organized as a legion. They were just 5 to 16 cohors equitata leaded by 2 equestrian prefects. And also their cohors structure was not normal for a legion. But, in late empire this new structure became somewhat "normal".

The central cohorts in Rome (praetorians, urban cohorts and vigiles) were of the new type. Finally standardized by the reforms of Augustus. Like the Auxilia was standardized during this military reform. This new type of cohors, ala or equitata was leaded by a prefect or tribune. The legion remained structured traditionally with just minor changes. And without a clear cohort commander. At least we don't know about him.

Yet, apart from the chiliarchos part which also appears, albeit seldomly, also in connection to speirae, there is another rank that crops up in relation with the speira, that of the praepositos, twice linked with the speira and a number of times independent of it as in an ile or a legion and I am inclined to believe that it was not a commanding rank but some honorary position.

The praepositus was the temporary commander of a vexillatio (detachment). It could be 30 men or 3000 men combined for a certain task or campaign. Another term is princeps (vexillationis), curator, curam agens.

Now, an interesting fact I came across was that I found no speira (cohort) numbered over 6.. Any insight? Shouldn't there be 10 cohorts in a legion? When looking for latin inscriptions, I think that there are multiple with numbers greater than 6 but I unfortunately cannot check them...

Thats fully normal for auxilia cohorts. There was often more than one with the same name showing their origin e.g Cohors I Batavorum .... Cohors VI Batavorum, or as high as XXXII voluntariorum c.R.
Could be any number, but they were not combined to a legion. Every auxilia was acting independently and reporting directly to the province governor, if not attached to an exercitus (army) during a campaign.

Finally, regarding the legionary cohorts, I will name some of the legions in the epigraphs and you can look into them :

eparchos of the 1st speira of the Apamenoi of the 3rd Gallic legion

chiliarchos of the 1st speira of the Ulpia Petraea legion

eparchos of the 1st speira of legion Flavia

eparchos of the 2nd speira of legion Claudia

eparchos of the 5th speira of legion Gordiana

Well, these sound like legionary cohorts. Are you sure you translated it correctly? Does it really mean "of legion" in greek? I just checked one of them, the Ulpia Petraea you listed and found:

IV Ulpia Petreorum
V Ulpia Petreorum eq
VI Ulpia Petreorum

These 3 are auxilia cohors; again no legionary cohors. Did your greek source mean the Cohors I Ulpia Petreorum?

Not knowing Latin, I just looked for the terms in question and here are some results. What do you make of these ?

1. Corinth 8,3 135
Saronic Gulf, Corinthia, and the Argolid (IG IV) : Korinthia

C(aio) Caelio C(ai)
fil(io) Ouf(entina) Martiali, praef(ecto)
coh(ortis) I Raetorum quae tendit
in Raetia, trib(uno) leg(ionis) XIII Gem(inae) quae

...

These again are all auxilia cohors, no legionary cohors commanders. This guy was Praefectus cohortis I Raetorum (an auxilia) and then was promoted to a Tribunus legionis XIII Gemina, without mentioning a cohors number, because he did not command a cohort in the legion.
A legionary cohort commander would be something like "Tribunus cohortis II legionis XIII Geminae". I doubt, you will find one, because no historian did so far, afaik.

Regarding the tribunus issue, I saw that normally, when the rank is given independently of a cohort it is given as tribunus militum legionis (although a number of times I found it as tribunus legionis). In contrast, when linked with a cohort, it is always tribunus cohortis and never (as far as I checked) tribunus militum cohortis. Do you think that these might be different ranks/positions?

Roman epigraphs are sometimes confusing, especially if the space on the tombstone is rather small.
Tribunus Militum and Tribunus Legionis could mean the same. It is a Tribune of a legion.
Just Tribunus militum could also mean a praetorian tribune, if clear looking the context.
Tribunus cohortis always means a leader of an auxilia cohors.
However, there was a ranking amongst tribunes depending on the rank of the unit the tribune leaded. A tribune of the praetorians or of an ala millaria was higher ranked than a tribunus cohortis or tribunus legionis. Also in late empire there were multiple ranks of tribunes.

Regarding the equestrian career there are 4 ranks (until the Flavians just the lower 3)
1. praefectus cohortis (quingenaria)
2. tribunus militum legionis or tribunus cohortis (millaria)
3. praefectus alae (quingenaria)
4. tribunus alae (millaria)



So, I am afraid, as insightful your information about greek sources and terms are, they all treat auxilia cohors not legionary cohors.