I am assuming you are talking about 20 gal long as a regular 20 gal or a 20 gal high. Either way, I would add new sand on the bottom of the tank first. I personally like to take eggcrate and lay it on the bottom. (I guess I dont' like the thought of rock on bare glass). Setup my live rock, then put in the sand. Since you have cycled rock and good quality live sand, I would have the live sand covering over the new sand. If you go the other way, you can cycle your tank again.
I personally would wait just a bit prior to adding more live rock. Just see if your tank cycles, unless it is already cycled live rock. If not you may wipe whatever you have in the 12 gal. (I am assuming you have corals and fish).
Backpak style filter/skimmer. I would hazard a guess that most responses will probably be the CPR BackPak. I have never used one preferring in-sump designs, but they and the Aquarium Systems
Seaclone skimmer seems to be the most popular. I have used a seaclone and was generally pleased with it even if it did seem a might tempermental. There is a another brand called the Prizm. I don't really like them for skimming. They do skim but you have to adjust them constantly. The cpr backpak is the only filter skimmer combo I would even consider as I have had skilters as a piece of junk.