Seniority

[Frank]

I had assumed that a Decurion would command the small cavalry contingent that was part of a legion.

We know of an Optio Equitis Legionis. So the romans used the traditional terms of the legion for this small cavalry unit of couriers and explorers. Afaik there is no evidence that they were commanded by a centurio. For administrative purposes they were perhaps on the payroll of the 60 centuries anyways. Perhaps one of the tribunes commanded them supported by this optio.

Were those all seconded from a cavalry unit? I wasn't sure if perhaps a centurion would command any Speculatores within a legion or a Decurion.

No, the Equites Legionis were soldiers on the payroll of the legion. Perhaps you mix that up with the Equites Singulares Consularis or Equites Singulares Legati Augusti pro Praetore. This was a unit of about 500 men seconded from the alae in the province. They were used as bodyguards of a province governor (Legatus Augusti pro praetore, Proconsul/Proprateor or Procurator) and for special deployment. An ordinary Legatus legionis had usually no Singulares.

Speculatores are something different. They are a kind of secret police force acting for a povince governor. They are high rank senior principales seconded from the legions. Higher in rank than an optio or a beneficarius consularis but not higher than an optio spei or a cornicularius consularis.

If you are interested in the NCOs of the legion you should look for some articles of David Breeze. He is the expert, if it comes to principales.

The auxiliary 'primus Pilus' I was really thinking the senior centurion within the auxiliary unit.

These guys are called Centurio Princeps or Decurio Princeps. Their function is comparable to the Pilus Prior of a legionary cohort. But not comparable with a Primus Pilus Prior of an entire legion. A Centurio Princeps or Centurio Pilus Prior is still a centurio of a cohort a ordinary officer. A Primus Pilus and probably other centuriones primi ordinis is an high officer (field grade officer) like tribunes and praefects and member of the generals staff. A Primipilus Iterum is even higher, perhaps rather comparable to a legate.

So an Auxilliary Decurion would outrank a legion Optio Spei? However a legion Centurion would outrank an auxilliary Decurion?

Yes an optio is a principales (NCO) and duplicarius (2 times salary). A decurio is an officer like a centurio and gets 5 times the salary of an equites.
A centurio legionis (15 times salary) is always of higher rank than a centurio of a cohort or a decurio of a turma. A decurio alae has the same function in the cavalry a centurio cohortis has in the infantry. The turma is the century of the ala, even if just 32 men strong. But legion is always of higher rank than auxilia/ala. However, I am not sure, if a centurio princeps cohortis could come close to a lower rank centurio legionis.

Just as a quick aside on the "acting centurion", it is more that the centurion role is performed by someone of another rank, rather than the individual's rank temporarily changing?

If a centurio is dead or out of town, the optio commands the centuria until the centurio comes back or a new centurio arrives. He is still an optio, and nothing changes with the optios rank. It is that simple. If an optio was already designated for centurio and is waiting for final promotion, he is called optio spei, but he is still 2nd in command after his centurio.

Perhaps you mean a vexillatio? If e.g. an optio is commanding a unit of 80 men as fort garrison at the border, he is called Praepositus Vexillationis. But his rank is still optio. Praepositus is just a function, not a rank.

Well, if somebody is commanding a numerus (irregular but permanent units in early and mid empire) as Praepositus Numeri, it sounds like a kind of rank. But this job is usually done by a centurio or higher anyways. So again the rank is centurio not praepositus.