Per Joyfulnoise and others, I too strongly recommend a separate pump for the waterfalls/sheer descents if you can, particularly if you are going with a VS speed pump for the rest of the pool. Although it's a matter of personal taste, a trickle or less vigorous water fall looks sort of feeble to me...almost looking as though your pool system is not quite functioning right.
Of course, low water speed/volume will have a good or bad appearance depending a lot on the design of the waterfall, spillway,etc. With mine, anything less that full speed creates an uneven, broken up, somewhat displeasing cascade water fall pattern. Just saying....