I don't separate my pups until they are approximately 6" tall. Many people won't separate them at all, preferring to let them continue growing along side the dying "mother" plant. I do that with my mounted plants, removing the dying leaves 1-2 at a time until only the pup(s) remain.
I use a well-draining potting media. Keep in mind, the major function of the bromeliad's roots are anchorage, not water/nutrient uptake.