As for Fox's D-air, it does more damage if you don't fastfall, but you have more combo opportunities if you fastfall the D-air.
Fast falling the dair does not give you any more combo opportunities. Non-FFed there are 6 hits, FFed there are 3 hits. It doesn't matter how many hits are in between; the last hit is always going to have the same properties. No change in each individual hit (beyond sheer amount) will produce no change in overall combo potential. There are really only two key things all Fox players should know and remember about dair.
1.
Not fast-falling your dair can sometimes be a terrible idea. More hits on the dair means more general air time. More air time means more time for your opponent to react to a hit and d.i. Going back to what we know about non-FFed dairs - 6 hits is plenty of time to attempt to d.i. out of the aerial and force the shine to whiff. If this happens, you might be eating a dsmash and losing a stock (via edgeguard/knockdown follow-ups).
Fastfalling the dair almost always guarantees your opponent will not randomly slide out of the last hit and be too far for the shine to hit. This happens alot with Sheik, Marth and Falcon, among others. My advice would be to always fastfall it so random d.i. doesn't thwart your combo attempts.
2.
More air time can sometimes be a good thing as well. You've all seen that dair -> d.i. backwards (bait shield grab) -> fsmash thing Falcos do, right? Well, with a SH non-FFed dair, you can get relatively similar results. Using the huge active hitbox of a single dair in combination with backwards d.i. - non-FFed dairs can sometimes net you grab or usmash ops that would otherwise be extremely difficult to obtain. This works well vs. shield loafters (Peach, Jiggs etc). Just don't overuse it because its extremely susceptible to SH aerials oos by alot of the cast.
Either neutral SH and perform the dair on a shield and d.i. backwards, or SH backwards and perform the dair. If you hit the lcancel, you have more than enough time to punish the grab your opponent thought they had on you.
Extra information:
-Dair is great when attempting an edge-guard you think you might otherwise flub. Ledge-hop dair pushes your opponent below the stage and right into shine spike territory. WDing back onto the ledge immediately after gets you a free edge-guard.
-Dair ***** crouch cancelling. No knockback on any of the hits means no knockback for the "CC" to cancel. If your opponent is holding down and beating your nairs and bairs, pull out your laser and start looking for dairs to start combos instead.
-Dair -> grab combos on any character that isn't Fox/Falco, Bowser, Jiggs. It will still combo on these four mentioned characters, but shine, Up+b, and rest are all fast enough to come out in between the dair and the grab. (Jiggs don't spam rest like that though, so don't worry) If you're feeling shaky on the dair->waveshine->grab, or simply do not have enough room (on a platform or near an edge)...this is what you should go for.
