I am currently working on the A&A figures, and like you, had many questions about historical validity. Especially the use of the rectangular Scutum. Since I wish to cover the entire period of the Third Century Crisis, I am more flexible to include the above scutum alongside round and oval ones. However, if I were you, I would consider the following: the famous Dura-Europos shield, found in the city that bears its name, has recently been dated by Yale University to be from the mid third century. It was found in what may have been the remains of an armory alongside 2 other rectangular and three, if memory serves me correctly, round shields. Now, Dura Europos was sacked by the Sassanids in 256 CE, and never re-inhabited. So, that means to me, that those rectangular shields were still part of the inventory in 256. So, it is more than feasible that Aurelian's army had units so equipped. Maybe you can represent that by giving the Rectangular shields to raw cohorts, or go the opposite end, and reserve the 'ancient shields' for the veteran cohorts. I think I am mixing mine.
Hope this helps you a bit,