Arsinoe was captured by the Romans, but not strangled by Cleopatra's guards in Egypt as shown. She was taken as a captive to Rome in 46 BC and remained alive, living in a Roman temple until 41 BC, when she was executed on Marc Antony's orders.
The legionaries are dressed like the Imperial Legions with lorica segmentata armour (plate in sheets held together by joints) and not as the post marian reform units they really were at that time that would wear a sort of chainmail armour or "Lorica Hamata". It is contested by some historians even that the Lorica Segmentata never reached widespread use and even maybe only functioned as a ceremonial type of armour.
In any case the first known (minor)a ppearance of this armour is in 19 BC, some time after the events portrayed in the movie.