The military structure of the Legion is copied from imperial Rome. Cohorts, Centuria, Contubernia, et cetera -- all based on the Roman Military structure. When Phoenix says that he has two Contubernia at his command, we know that he means he has roughly 16 legionaries and 4 slaves/reserve troops at his command because of what the Roman Contubernium consisted of, and that is 8 legionaries and two reserve troops/pack mule type men.
Praetorians are Caesar's elite guards, just as they were the elite guards of Roman emperors. In fact, every title in the Legion -- Centurian, Decanus, Frumentarius, Legatus, et cetera is copied right from ancient Rome and they lead the same group of men, a Decanus leads a Contubernium in the Legion, and they led Contuberia in Imperial Rome as one example.
The Legion's military tactics are also derived from the Romans, with the first wave being recruit legionaries, the second being Primes, and the third being Veterans. This was taken from earlier in Roman history during the time of the Manipular Legion, when the weakest soldiers known as Hastati were the first wave, then following them were semi-experienced military men known as principes, and then following them in the third wave were the veteran, experienced troops known as triarii.
So Ceasar combined different military characteristics from different periods in Rome's military history to create an army that he deemed applicable to fighting in the post-apocalyptic world. The structure of Legion>Cohort>Centuria>Contubernia was from later in Rome's history than when they organized their soldiers into the three wave attack of Hastati, principes, triarii. But Caesar only did that because his army isn't large enough to fit with the organization that went along with the Legion, Cohort, Centuria model, where in ancient Rome each
Cohort was essentially a wave in the attack -- the first being the weakest and the last being made of the strongest veterans. It was this way in the hastati, principes, triarii structure as well in ancient Rome, but Caesar (in New Vegas) was able to boil that strategy down to having individual contuberium act as the different "waves."