Dude, nothing would make me happier than being able to Fsmash Falco out of a chaingrab, but proper DI towards the end of the CG gets you out of it. At the lower percents, FSmash takes too long and they reach the ground, techroll and punish your endlag. It may work once on an opponent unfamiliar with the match-up, or someone that plain screws up (so keep an eye out for it, I've gotten ridiculous things like a completely not-guaranteed FSmash on Pit out of a Dthrow) but it's punishing a grievous mistake, and not reliable at all.
Bair is MUCH more reliable because you can read the DI and put Ike at the proper height real quick. If you're close to the right edge, it could mean a dead spacie anyway, and more often than not lead to the edgeguard. You want to slay spacies, learn to powershield, pretend you're Falcon and cheese HARD for a grab and be able to do the Uthrow chain -> bair in your sleep. This is also why Dreamland is not a terrible pick against spacies: The fear of camping is real, but Wolf's mid-range projectile is sub-par for camping (it is more a zoning, or pressure tool: How do you deal with a slow-moving laser coming at you with a Wolf right behind it?) and you have that nice, open middle space to chaingrab. V/s Fox it's a mixed bag of "I can survive USmash longer" v/s "I am more prone to having Fox rack up laser damage" but that's why you work on your wavelands, platform game and just good mobility in general. You want to be a stock ahead and then turn into Ivy: Don't play Fox's game, don't try to rush him down as he tries to SHDL you into USmash percentage, just focus on weaving between those infernal red lines and keep your eyes peeled to punish any mistake or sloppy approach; focus on putting Ragnell between you and Fox, cheese for a grab like before, etc.
Woo, that was a lot of stuff to put into one post, huh?