What do you mean by a "pleb?" Do you mean a plebeian? If so, all that a plebeian is is someone who is not descended from Romulus' patres, which is to say not a patrician. The vast majority of the senate and at least half of the nobiles were plebeian, and by the Principate the patrician order had essentially ceased to exist, having been artificially recreates by Augustus through adoption and the promotion of a number of plebeian families to patrician status. Cato, Cicero, Antony, Crassus, Pompey, Hortensius, Lepidus, Cassius, and Brutus were all plebeians. If you mean someone outside of the senatorial class, the answer is still yes. The equites frequently ascended to the senate, hence the term novus homo. Both Marius and Cicero were novi homines--all that was required for entry into the senate was election to the quaestorship and successful completion of your year in office as quaestor.

The requirements for appointment as a legate changed over time, even within the Principate. During the Republic legates were, like a promagistrate's military tribunes (after those for his four senatorial legions), more or less appointed directly by the promagistrate. There were technically no requirements, although naturally one would expect the post to go to one of senatorial status, though not necessarily a former holder of major senatorial office--Caesar's legate Labienus had only held the tribunate of the plebs. Under Augustus legates were generally of senatorial standing, usually being praetorians or consulars, whether legati legionis or pro praetore--in Egypt, however, legions were commanded by an equestrian prefect. Later emperors appointed mainly equites as legates, but the office was generally one appointed, as with most of the important functions of the military in the Principate, directly by the emperor