Giving the ability a shorter cooldown sounds appropriate. Quelling Strikes is typically only useful at one time: the very start of a fight. Tank enmity generation in this game is frankly ridiculous; a couple of enmity rotations and the mob is your friend for life, no matter what the DDs do to it. If a BRD or BLM is going to pull hate it will pretty much always be at the start of the fight while the tank is still gathering a group of mobs together and hasn't had a chance to establish firm enmity on all of them. Giving Quelling Strikes a cooldown that allows them to use it once per battle is appropriate.
In the case of bosses, Quelling Strikes is pretty much pointless after the first minute so it's not as though letting them have Quelling Strikes again will make a lick of difference. Again, no opportunity for abuse.