Technically, no, you can’t—free actions can only be made on your turn unless something explicitly says otherwise (as with talking). But this largely doesn’t make any sense, see this answer for more details on why I would say you can.
But even if you can, you can’t guarantee what your foe will do. If they recognize calm emotions for what it is, for example, they could easily delay their turn until you stop concentrating (which would have them come after that turn, so you would get one ally’s turn in), or ready an action until you stop concentrating (which would happen immediately, but only be a standard action). Even if they don’t recognize calm emotions, they can always do non-aggressive things to improve their side’s chance of success—even someone magically compelled to remain calm could see value in buffing their allies for a situation that they see as not yet dangerous but possibly threatening to become dangerous.
But sure, against many foes, it will be pretty effective.