Were the Marian reforms already in effect when Rome fought at Arausio?
Short answerNo. Arausio came first, and the emergency created by the defeat helped bring Marius back to institute the reforms afterward.
NotesThe army destroyed at Arausio was still the pre-Marian force. The battle took place in 105 BC, before the reforms. The catastrophe exposed weaknesses in recruitment, standardization, logistics, and command that Rome could no longer ignore. Marius's return belongs to the aftermath of the defeat, not to the army that fought the battle.