Hi and welcome on board.
First of all, this is a question for the Q&A thread of the forum.
Second, you could go and get some guides from the DL section that explain everything.
Third, overlapping: just set down the buildings you want, move them one on top of the other, add a terrain spike (a rise of the ground with smallest brush size beneath a building that elevates it) and while doing all that, press K and shift-K to alter the size of the particular building.
OK?
regards
the Elder
"imagination is more important than knowledge."
Albert Einstein