You can get away with OT opener as MT if
-the DPSes aren't very bursty (eg. no skilled BLM who don't use quelling in your party) or
-you have Ninja to shade walker you and smoke bomb the bursty guy
Shield swipe proc from Sheltron also give you an overhead on threat too.
As for MT rotation, PLD lacks super threat generator skill like Unchained, so if the DPS are catching up on threat the I will rotate between RoH and RA combo, refreshing GB as needed. Also dropped to Sword Oath once I'm in comfortable zone and no tankbuster in coming.