I keep trying to fast-fall after using neutral B on Zero Suit Samus, but either I can't see well, my timing's not good, or it's not possible. Can this be done?
Fast falling can only be done when you're in the air and your character is waiting for input. The exception to that is if you dair (Down + A in the air), you can fast fall simultaneously with the attack. But when you use moves like neutral B, you have to wait for it to finish before fast falling. You would most likely be on the ground already, unless you're double jumping.