DSB should be at least 3 inches for it to work properly. i believe i have about 4 inches.
live rock? people say 1-2 pounds per gallon. but i would think you would have enough filtration from the berlin and DSB that live rock amount can be set by how you want your tank to look like.
i've actually removed alot of rocks from my tank and into the sump because i sorta like the wide open area for my fishes to swim.