How were officers trained?

[popularis]

It must be this pre service training and study that qualified some young Equestrians for direct appointment to the Centurionate!

I'd go with 'preparation' rather than 'training'! But your point above about inexperienced officers being shunted away to minor duties to gain experience (which I believe makes a lot of sense for the military tribunate) could hold for centurions as well. However, this does raise a tangential question that's always bothered me: would young men of equestrian family who served as centurions have been subordinate to non-equestrians (e.g. more senior centurions who were not yet of equestrian rank)?

A lot of the structure of the legion hierarchy comes partly from the assumption that, for example, senatorial-ranked military tribunes would be senior to praefecti castrorum or the other military tribunes because senatorial status always trumps equestrian status. It would be interesting if equestrian status did not always trump non-equestrian status (although there are get-outs for this, particularly in the case of young men who were the sons of equestrians and not, perhaps, technically equestrians themselves).

Blue skies

Tom