Ok, for the aerials, Nair and Bair have a sweet spot. The easiest way to get good knockback is to fastfall it from above into them while using Fair.
Nair on the other hand has various damage output. If you hit them straight or from below, it deals max dmg (8%) and knockback, if you FF your Nair into your opponent from above it does less damage (5%).
Bair>Fair>Nair, however, against bowser in training, I needed about 180% for the Bair and 190% forn the nair kill. So the difference is noit too big actually.
As far as i know, Nair and Fair have 10 Frames of landing lag, thats about 1/6 of a second (60 FPS, then math...). Bair has... 18 iirc, but tbh i don't know this ecactly.
You'll probably be sad about this: Bair has no reverse hitbox. Even tho it has a big hitbox, and can even hit infront of Sheik, it always knocks the opponent the other way. However, both Nair and Fair have a reverse hitbox (you'll have to hit from below and on your back side, thats the easiest way). So as far as combos go, you probably have to work with weak and strong hits, maybe start of with a weak Bair and hit then with the reverse Hhitbox of a strong Fair. Or the other way round, weak reverse Fair into strong Bair. RAR will be your freind here.
Messing around a little bit I was able to set up some kills, however, training, and extremly hard to hit, so I don't know how useful this really is.