It is generally best to use a fairly dense pad under berber. We usually recomend 3/8" 8 pound. 1/4" pad works fine too but doesn't feel as good under foot. We stay away from using 1/2" pad under berbers as there is too much "up & down" movement.
As for scewing the floor down...., Is it loose? Or squeeking? If not just go over it.