Pullo's angry defense of the 13th Legion was typical of soldiers of the time. The rivalry between the various military units was fierce, and was encouraged by their respective commanders because it made their men fight even harder.
The death-blow that Vorenus delivers to the gladiator was a classic move, used in the arena for centuries.
Mascius claims to Vorenus that the veterans of the 13th Legion are angry with Caesar because he has not given them land in Italy. Vorenus, through negotiations with Caesar, gets the veterans land in Pannonia (a province northeast of Italy, mostly in modern day Hungary and Croatia, and not entirely under Roman control in Caesar's time). In reality, the 13th Legion was disbanded by Caesar in 45 BC and the veterans were indeed given farm land in Italy.
The backstory of the final gladiator was that he was a German, who was previously undefeated.
Vorenus' first client as a magistrate was a "piss-pot" dealer. This unsavory business was the placing of pots in public locations for the public to urinate in, so the urine could be collected and sold as a chemical ingredient (uric acid) in various industries, principally fabric dyes.