I think you are planning on too many fish for that size tank. And, using two different substrate materials is ok only if you want them to mix. Trying to keep them in separate areas means lots of work for you, if it is even possible, other than for short times. I suggest looking at some of the very simple aquascapes, that use mostly carpet plants, a few rocks, and nothing else. That will at least save some tank volume for the fish. A 12" high, 12" deep tank doesn't have a lot of excess volume.