In the hall of Senate, everyone should wear a white ("dealbata") toga, meaning they all are equals (one senator, one vote). Instead, they are wearing "laticlavium" with a large red strip - the patricians - and "angusticlavium" with a smaller red strip - the knights ("equites") - which should be worn just outside, in formal ceremonies. Otherwise, they wear the color they want. Cato was indeed historically dressed usually in black, but in the hall of Senate he had to be dressed in white like any other senator.
A talent of gold isn't an amount; it's measurement by weight, roughly equaling about 70 pounds. Marc Antony is given a half talent of gold, roughly 35 pounds, which he carries easily, and in a pouch. In addition, he gives a quarter talent, roughly 18 pounds, to Lucius Vorenus, again handing it over with little effort, indicating the lack of accuracy.
The warriors that colored their skin blue were from Great Britain, not Spain. The first known author to mention this Brittonic custom was none other than Julius Caesar in his "Commentaries on the Gallic War".
The first emperor of Rome was originally named "Gaius Octavius". When he was adopted by his great-uncle, he became "Gaius Julius Caesar Octavianus". That adoption only took place with the reading of Caesar's will. Nobody, not even his mother Atia in a moment of wishful thinking, would have addressed him as "Octavian" prior to that event.
When Vorenus and Pullo rescue Octavian, he assures them that Caesar will prove his identity. Octavian is eleven-years-old at this point and Caesar was away in Gaul for nearly eight years, so the last time he would have seen Octavian in Rome would have been when he was three. However, upon seeing Octavian Caesar recognizes him straight away.
In the battle against the Gauls, some shots show either the Romans or Gauls (or both) with their shields in their right hand their swords on the left. This indicates the scenes in question have been "mirrored" in post-production.