So I was trying this out, and honestly I don't think it's worth it, people can either mash out of your down smash or not, so to time the foot stool out of it is very hard, and the fact that foot stooling in this game is even harder than Brawl makes it even more difficult to do.
the thing is though, fast fallers can mash out of stun but it's really really dangerous, if they fair/dair they could SD.
I'm not saying this isn't EXTREMELY situational (when I do kill with it though its stupid hype), but if you want some kind of edge in a fox/falco matchup... well, here you go. BTW, the stunstool is actually amazing and easy to get with falco, since he often phantasms into the ledge and stun can always catch that, you could also just wait for the pop-up and f-smash them, which for higher percentages (or bad DI if they are trying to mash out) may be an unrecoverable knock back.
The saltiest way to kill a falco is to low-swing f-tilt him to his death out of a phantasm edge stun.
Also, I do have some honest fox matchup advice.
1. Get them off the stage and stun guard the best you can, unless they try and sweetspot from below, in which case you should just wavedash offstage then upB to edgehog
2. Never try and tether recovery if they can potentially get a shine-gimp, because they will get the shine gimp
3. u-tilt/u-air/upB/upsmash combos can be possible (on a stage like PS2) if you are a god at ZSS pillar combos
4. Laser will be reflected if the fox is at all competent, but they will have to hold a shine to get the reflect, so you can over-b fox as a punish if you dash-cancel your shot and follow the lazer, your over-b needs to hit before he reflects is my point.
5. bait them into your backslide pivot whenever possible (thank god her grab can pull people out of the air)
6. Retreat from fox's air approach (nair-planes and whatnot) with flipjump, if done correctly you can get a punish from it
7. Play on PS2 or any 2+ platform stage and up-smash/upB/upAir poke a lot, sure an SDI out of U-smash is inevitable but for shutting down approach from above there is no equal.
8. Be under fox or very far away from fox onstage, anywhere else is dangerous.
9. My CQC ground game very often becomes SHFFLd Bairs and Uairs
10. If you feel like you need to shield pressure (because you are crazy), jab-dtilt is pretty good, only a small punish window