Based on what I got from TBS, there would be no possible way to fit a 15 gallon TBS package in a nano cube. I ordered the 10 and the aquascaping was difficult; I ended up taking a few pieces of rock out of the cube and into another tank because it was so full.
I think I killed my first attempt at a TBS nano because I tried to use every bit of rock & sand they sent, and I didn't leave enough water. A 12 gallon nano cube only holds about 8 1/2 gallons when completely empty. Add 20lbs of rock and 10lbs of sand, and I bet you've got less than 5 gallons of water in your '12' gallon tank. In my experience, that wasn't enough water volume to properly cycle, and maintain, that much of a bioload.
I love my nano cube, and I think it's doing pretty well, but I've only got about 5 lbs of sand and 10-12 lbs of rock in there.