Do pay attention. The topic was the draftees from unfree manpower pools, not the usual recruitment methods.It wasn't academic distinction. To former slaves, it was improvement. They were unfree, unpaid labourers, who might also suffer from food shortages and bad housing. In auxilia, they were paid, armed, fed, clothed and housed, when not on march...
Really? Conscription is still compulsory in most of world, in case of war... that doesn't make one or other country slave owning countries. Anyway, for most part auxiliaries were volunteers.
And while involuntary enlistement was a step up from slavery or imprisonment by any yardstick it was nevertheless de facto just an exchange in which form of authority and discipline you were obliged to submit to or else. Such men were involuntarily subjected to the embarracked lifestyle and harsh discipline of the military for decades without any of the usual niceties of even conscription and weren't exactly free to quit at their discretion; while certainly an improvement over their prior circumstances (all the more so in terms of future prospects) it is hardly an exaggeration to say that in real terms they remained effectively unfree until discharge.
I have no idea why you're limiting the definition of "officers" in the Roman army to the senior officers (tribunes and above, always of at least the equestrian class), nevermind now tying it to social class - not to mention that centurions of the primus pilus rank were automatically elevated into the equestrian class anyway. Actual historians certainly don't and centurions are generally considered broadly analogous to modern captains (a commissioned officer rank).Officers of Roman armies were comites (precursor to counts), legates, tribunes.., drawn from elites, patriacians and equites. Their authority was simply far beyond that of centurions, who were drawn from footsoldiers. Legates and tribunes, did have some instructions in strategy, and tactics before entering army, centurions went from bottom, and learned through service, like modern non comisioned officers. They had more of authority than modern warrant officers, and better social standing, still, they were not considered part of officer corps by officers of Army, and were of different background, upbringing.
And the Romans had no more systematic, formal officer training than anyone else before about the 1700s (which is about when the Europeans first started dabbling with the idea) - as usual the upper classes were sort of just expected to pick up the necessary leadership know-how holistically from their upbringing (ie. reading/hearing accounts of bygone campaigns) and maybe if they were lucky from serving as aides to active commanders, while those coming up through the ranks one way or another were down to pure "learning by doing" insofar their abilities allowed. As you might imagine the results were... uneven. (The rather complicated cursus honorum career progression of the Roman elite, which repeatedly rotated them between civilian and military posts, probably didn't help...)