Promotion in a Vexilatio

[Frank]

If there were 60 centurions per legion, plus and at any one time there were a good percentage dead or injured, plus another percentage potentially travelling back and forth (if that theory is correct), at any one time the active number of actual centurions could have been very low, especially by the end of campaign season during a time of conquest.

Theoretically 59 not 60. 9 x 6 plus 5 for the 1st cohort. But usually much more, called supernumerarii. We know of cohorts which had up to 11 centurions even if max 6 of them could command a centuria. The others worked in the staff of the legatus augusti pro praetore, the province governor and leader of the entire exercitus (army of one or more legions plus auxilia). A lot of centurions as well as tribunes, principales and ordinary soldiers (singulari) were busy administering the province. Not all of them were supernumerarii. If you look to the daily report of any unit in the roman empire, you always see a group of soldiers detached to the governors HQ. The legati augusti pro praetore also detached staff for propraetors and procurators of provinces in the neighbourhood, which had just a few auxilia or even no units at all.

We can be sure, that a lot of soldiers of Varus' legions survived the glades variana in Teutoburg Forest, because they were in Belgica or anywhere else in Germania busy with administration.

I assume the same would hold true for promotion etc of centurions with auxiliary cohorts and marines. Later on the same would be true for Praetorians and urban cohorts?

There is technically no difference between centurions of the different units. However, the rank (and payment) is different. Legionaries have a higher rank than auxilia soldiers and they are higher than fleet soldiers. Looking to the cohorts in Rome the praetorians are highest rank, followed by the cohortes urbanae and the vigiles. That has something to do with tradition and social rank. In most auxilia non-roman-citizens were allowed. In the fleet and the vigiles even fredmen. Some centurions of e.g. the auxilia were provincials but others optiones and other principales from the legions promoted to centurio of an auxilia as a first step of their further career.

It also raises another question. Many legions and Auxillia were raised far from Rome, for example IX Hispana. .... I assume before Augustus that the Imperateur would deal with field promotions and the appointment of his own officers?

In the late republic, the commander of the army aka propraetor/proconsul was responsible for promotions. Depending on how far you go back in roman history you will see more centurions elected by the soldiers. Regarding the higher officers, most of them are selected by the commander. They are often friends and clients of the family. However there was a need to give jobs (e.g. tribune) to members/clients of other families, too. Tribune was part of the cursus honorum. And sometimes legates were appointed directly by the senate. The quaestor was even elected by the people of Rome, but usually not a political enemy of the commander.

Technically, there was no difference between the republic and the empire if it comes to promotions of centurions. But, there was just one "superproconsul" and commander, which was the empire himself with his imperium proconsulare. All legati augusti pro praetore were just subcommanders without imperium. Therefore it is still the same procedure: the commander decides, not the subcommander. Well, that brings up an interesting question: could a proconsul of a senatorial province (e.g. Asia) promote his centurions? Because he had a proconsular imperium of his own given by the senate and usually a few auxilia. But honestly I don't know, and practically no proconsul with a brain acts against the emperor.

Looking to new legions, it must have been a great day for all the principales of an army, when the proconsul raised a new legion. Lots of them became promoted. Sometimes veterans were called back into service with now the higher rank of a centurio.

After Augustus, were all officers, not just centurions, appointed by the emperor or at least the central command

That was my point. Regardless of this ceremony and how mandatory it really was, the important point is: Centurions and higher officers were promoted by the emperor! Even dismissals of every single soldier needed an official document from Rome. A copy of every dismissal was even stored in Rome. The roman empire was the biggest buerocracy of the world until the European Union was established. Well, not counting the Chinese Empire of course.