I don't think Puffs dair as safe as those 3 moves mentioned before. Though the optimal AC for MKs nair is almost irrelevant as it's way too precise to do, but be my guest and try.
Oh and please explain the floatcancel thing. I know what I am talking about and it doesn't exist. It was like the first thing people mentioned when Brawl was released, floatcanceling is gone.
1. Dash (OR DASH HOLDING DIAGONALLY DOWN)
2. Hold down/input press y
3. do steps 1 and 2 but press a while holding forward and press A
4. You are now using her fair on the ground with her dash momentum because instant fair no float
5. Cancel fair on shine
6. USE ANYTHING SHE HAS even taunt can be done ON shine.
7. Notice there is 0 frames of lag
8. try this now with uair
9. Cancel ON hitbox
10. Notice there is 1 frame of lag that can be cancelled with her jab
11. Proceed with bair, but cancel bair with bair across the stage 0 frames of lag
12. Notice this doesn;t work with dair until after the hitbox.
13. For the lolz, input that newly attained ground fair 100 times to go across the stage.