Hey Brooks,
Thanks for your interest in my reef tank. As far as my live rock...I have about 51 pounds. It is all Figi live rock. 34 pounds were bought uncured and 17 pounds were brought over from my old nano reef. Not all 51 pounds are in the display, I also have some in the refugium and HOB filter. You are correct, I have about 1.25 pounds per gallon. My belief is to use the best quality live rock, vs the highest quantity. Some tanks go as 2.5 pounds per gallon. I am planning to have my corals grow out. Especially the Staghorn corals on top. So hopefully, there won't be any "unused" area in the top of the tank. As far as your denitrication question, you will acheive denitrification as long as you let your tank cycle and get high quality rock. Over time that aerobic and anaerobic baceria will do really well in the pourus rock. You'll be able to sustain a decent bioload. It's always best to add fish very very slow in my opinion so that there is no shock to the bacteria. Patience is the name of the game it seems with any tank I've had.
As far as flow, I have a wavemaker that has 4 powerheads on rotation. 2 are maxijet 1200's and the other two are Rio powerheads...I also have increased flow from the HOB filter....Well, hopefully this wasn't a long rant...and I helped answer some of your questions.
Also, good luck with your 20 gallon tall. I'm sure you can do lots of searches to find others with this same tank.