Let's pretend for a second there is a legionnaire named Marcus and a centurion named Augustus. Would the legionnaire refer to his superior officer as "Centurion Augustus"? Or would he say "Domine Augustus"? (Domine meaning "master" or "sir" in Latin). Or would he say "Lord Augustus"? Or would he just say "centurion"? Or is there some other term I'm not thinking of? Reversely, how would Augustus refer to Marcus? Would he say "legionnaire" or "legionnaire Marcus" or just plain "Marcus"?
I can access the work cited by njuffa in a comment, R. Ferri: Language Use in the Roman Army, here (it says you have to pay, but if you scroll down, you can see at least enough of the text to get to the forms of address).
It says under chapter 3, point C (p. 162):
The standard form of address is domine. Letters from the equivalent of non-commissioned officers (principales, beneficiarii) to their superiors use the bare domine […] There are cases in which the simple domine is paired with other honorifics.
Of those other honorifics, two somewhat curious cases where the superior is also addressed as rex (king) are cited, and one Greek example, so I would not put too much weight on that.
A short note on the Latin: The word is dominus and is declined according to context, so when directly addressing someone, it would be domine (vocative), or in a letterhead it would be domino (dative). While the cited work makes no mention of this, in letterheads the word would definitely be combined with the name, and we can see this in one of the examples the author cites, if we look up the whole quotation:
Cl(audius) Tiberian[u]s Longino Prisco domin[o] et regi suo plurimam salutem
(Emphasis mine.) Please remember that, if it used together with a name, the name must be in the same case, so it would be Domine Auguste if the interlocutor is indeed named Augustus (Marcus was a very common Roman name, Augustus definitely not.)
Dominus means “master” or “ruler,” and was also an honorific for later Emperors (and in Christian Latin is a standard word for God, like the English “Lord”), but it was also a mildly polite greeting for everybody, as explained in this previous answer.
Regarding the opposite case (commander addressing a simple soldier), under point A, Ferri says:
both miles and commilito are used
Miles, of course, means “soldier,” and commilito means “comrade, fellow soldier.” As one might imagine, the author reports that miles seems to have been used when shouting orders, and commilito more in cases where superiors wanted to emphasize their own soldierly status, in particular when giving encouraging speeches, but also in relatively neutral cases.
No example is given in which any of these is combined with a name, but by its nature the evidence rarely contains addresses of individual soldiers. When commanders addressed soldiers, they will also often not have known the individual's name. In my opinion there is no particular reason why miles Marce should not be possible.