Me first calls the other attack, so it should act as if your Pheromosa is using that attack and use your Pheromosa's item when calculating damage. I'm pretty sure you get choice locked to me first, not the move that me first calls. I don't know how it works with the metronome item. (ie. If Pheromosa uses bug buzz turn 1 and uses me first turn 2, and me first calls bug buzz, then I don't know whether that gets the metronome boost)