Isnt it accepted that the Kalkriese type of lorica segmentata was in use from 20BC to about 50AD? They found the breastplate dated 9ad from Teutoberg Forest so if the legions had them then, the praetorians who were better equipped should have had them some time before.(unless of coarse the less important guys would have the nice armor first which wouldnt make any sense) So maybe a praetorian unit having it wouldnt be far fetched.