There isn't much evidence for military protocol in the Roman legions. Legionaries weren't called 'soldiers' before Augustus (they were referred to as 'Brothers'). Specific ranks would be a little tedious so broad categories are likely, Centurion, Tribune, Legate. What you will have to become aware of is that the Romans may well have not used analogous behaviour to modern armies (a typical Hollywood or literary ploy).
Saluting has been debated for a lo/ng time and most people feel comfortable with a modernesque protocol, but the sources do not mention saluting outside of honouring a commander as opposed to recognising his superior rank as we do. What this means is that ordinary salutes may not have happened, but that soldiers who approved of their commanders may have deliberately or spontaneously saluted them as something.
Incidentially using the word 'Domine' might well have been seen as 'licking the backside'. In fact, such language does exist in letters recovered from Vindolanda. It does not refer to rank, but names the recipient as 'Master'. That's a very subordinate form of phrasing because it infers that you are indentured to the recipient in some way. Legionaries swore an oath of obedience - this was necessary because obedience to another man is the same as slavery, and Roman soldiers would not tolerate such associations. They were free citizens, soldiers or not.
The upshot of this is that I am thinking in terms of names being more contextual than actual rank titles in many cases. Remember that in the legions, loyalty is fixed toward individuals rather than offices.