Boadicea and Julius Caesar could not have fought, since Gaio Giulio Cesare was assassinated in 44 BCE and Boadicea lead her uprising against the Romans ca. 60 CE--a century later.
Xena must have been a truly remarkable person for not only was she present at the Trojan war, as we have seen in a past episode, but she also had the honor of knowing Julius Caesar. Considering that the Trojan war is historically placed at about 1194 BC-1184 BC and Gaio Giulio Cesare's life from about 100 BC - 44 BC, for Xena to have survived over 1000 years is utterly amazing! Talk about your well kept woman.
In Caesar's tent paintings are visible that are stylized based on a later date than the era that is represented in the show.