the answer to all your questions are in that thread, but let me just give an actual application of this in a match:
say your opponent is at ~80%, and you'll uair him as fox for the vertical kill.
sometimes the second hit of the uair won't connect, the opponent will pop up a little bit and what do you do
this is where some people forget about stale moves and try to uair once more, since most of the time it's a free continuation of the combo.
unless it's way, way past the kill %, that second uair won't kill, because it's now stale.
the same goes for edgeguarding vs mid-low %s, sending the opponent offstage with a string of nairs and fairs (finishing a combo), etc.
your last hit of the combo should be the strongest
related to that, find out what are your character's strongest options on the ground, in the air, etc. this is relevant because people usually get accustumed to whatever and use that all the time when it's literally the worst option in terms of knockback. example: falcon bair. bair is the pretty much the worst option, but for various reasons, falcons will bair to finish combos and to send opponents away. (f punch > hi-fsmash > f-smash > hug > nair > bair)