Really, everything comes to preferances when it comes to control setup, as well as what works for your character the best. An example would be setting a shield button to attack so you can boost smash.
You do SHFF'd Uairs by short hopping, doing the Uair, and then immediatly holding down, like you're fast falling. You want to use fast-falled attacks whenever that is the best option, sometimes just a Uair can take care of the problem, although generally you want to get to the ground fast, so you SHFF. It can be easier if you have aerials set to C-stick, as you can just smash up on the C-stick and smash down on the control stick. The same applies for every attack, that's why when you do Dairs with certain characters of the stage you never hold down the control stick and instead just enter the command for Dair.
The list was basically from the most technical to the least technical out of all those. And ZSS isn't that technical compared to the rest of those, but hey, the character you use is ultimately your preferance.