Apologies if this is too basic, and feel free to delete, but I am curious to know how Romans would address a person of higher status - not a slave his/her master/mistress - but, for instance, a wage-labourer to an employer, a shopkeeper to a high-status customer, any "commoner" to an aristocrat or, indeed, a slave to one who was not his/her owner?
I don't know the literature well enough to give you an authoritative answer, but I believe it's fairly well established that forms of address did not vary based on rank, class, citizenship, etc., until the 4th century A.D., when people started addressing the emperor as "vos." Roger Brown and Albert Gilman offer a few possible explanations for the development in "The pronouns of power and solidarity," in T.A. Sebeok's Style in Language: by that time, there were two Roman emperors, one in Rome and one in Constantinople, but they were administratively unified, so words addressed to one were actually addressed to both. Emperors and kings are also the summation of their people and can speak for/as them, which is why kings and queens say things like, "We are not amused." Or it could have just, you know, happened. In any case, eventually the usage broadened to include other power figures.