Yep. Though, on larger characters like Rosalina you can input the zair immediately after your short hop, which is probably better to do or you could get hit before you get the chance to zair the opponent.
Well, I almost always land-cancel my zairs because it cancels REALLY fast, assuming that you didn't airdodge before the input. Rising zairs are only good on reaction-- so this works in very rare situations, since most of the time, you're using projectiles to zone your opponent out anyway. In Brawl, I've seen people stop phantasms/illusions with zairs.
If you read it right, boomerang does the same thing, with a longer duration hitbox, so there's a longer window of time where they can run into a boomerang. Boomerangs start up slower than zair, though.
I've also seen people interrupt Meta Knight's glide with zair and boomerang, which of course, does not exist in this game. So I guess boomerang/arrow/zair can be used for opposing your opponent's bum-rushes.
The little amount of lag with zair also means that it's really great for spacing, and a followup.
I remember Yackabean saying somewhere that zair -> grab works nicely.
Do you guys still use zairs for gimps? Everyone's ridiculous recovery in Smash 4 makes it seem like zair gimping isn't as applicable as it was in Brawl. The reduced lag time in the air compared to Brawl zair is nice though, although it seems that Smash 4 Toon isn't as floaty as he was in Brawl, which kind of negates that property...