I think you would be ok with one pump, as long as you put in plenty of valves to direct water flow how you need it. I set my values up so i could pull from MD or Skimmer or both. For returns, i can push back out to steps, walls, deck jets, water fall. I can turn on off any. I do not have the waterfall completed yet, but I stubbed up a 2 inch line for it. When i run the deck jets, I turn off the steps and turn down the wall returns, I get plenty of flow from the deck jets. When i have the waterfall on, I suspect I will turn off deck jets and wall returns and divert most to waterfall.
bottom line, lots of valves for flexibility is key in my opinion.