Your 15 is looking good, but I agree, I would put more live rock. When you add more rock, your tank will go through a small cycling again.
Im running a 12 gal nanocube, 15lbs of live rock and and 10 lbs of live sand. I also run a fuge in one of the back chambers that has a deep bed of LS, some LR rubble and some macroalgae.
At work, I have a picotope 3 gal with no fuge. Its running great. Nothing in the stock HOB. I only use it for flow. I also added a small elite powerhead type filter for more flow. Im not running any floss, carbon or anything. Just 50% water changes weekly.