I'd get two, and yes that's plenty of filtration. The tank you are getting is 6' long, so you can have intakes/returns and both ends and get good circulation throughout the tank. Once it's cycled, you won't read ammonia or nitrite as long as you stay on top of water changes. I do 75-80% weekly.
If you're just running the Fluvals, then you may want to consider adding aeration. It would be pretty easy to set up an air pump with an air stone in each corner, and the tank would be plenty oxygenated.
FX-6 maintenance isn't that bad. Just don't put anything in it that requires being changed like filter floss, carbon, purigen etc. I have bio media in the bottom 2.5 baskets, and the very top I have Bio-Foam. It's very low maintenance.
Would an air stone be okay for the Piraya? And would it give enough oxygenation if I put two on each side? Do I need both an air pump and the stones? Or can I just buy stones? As you can tell I'm very new.
As well, I was thinking of using a powerhead but I have heard they can be dangerous if they short out.
What all options do I have for oxygenation? Air pump, powerhead, air stone, Aquaclear 110, What else?