Fox can crouch 'till more than 25%.
Needle, grabs, utilt to grab are good option against spacies
Depending on the situation, tilts or grab can be effective as counterattack.
Szadek :
Work on your DI : with a good DI, Falco won't combo that hard on Sheik.
At first my DIs were crappy, and Falco goes for ~60% combo.
Then I work on my DI, so now it's much more like 20% or 40% depending on the player.
Also, when I play Falco, against crappy sheik i can go for 40 or 60% even if my Falco is bad :/ but against a good Sheik player (Darko) I always end with just 2-hit combo and he escape (and kill me btw)
DI are very important in this matchup.
Falco's bair is very easy to DI.
It's pretty hard for Falco to combo Sheik without platforms. If you're not scared of lasers, you could go FD. (To avoid laser, Shorthop to waveland, WD OoS, Crouch are good options)
Also, if you have video/more precise problem, people help you a little more.