Uthrow->usmash is guaranteed at low->mid percentages on fastfallers (just follow DI). Dthrow to usmash is guaranteed on no character at no percentage (that I know of). I'll test it out some more, but I'm pretty sure anyone can DI out of the usmash after a dthrow. You can, however, techchase them after a dthrow (at lower percentages on non-superlight characters) into a usmash or grab.
Additionally, on the falco matchup, at lowish pika percentages play primarily near the edges of the stage with your back to the edge. This way, if you get a grab in, it leads to backthrow, then you can jump out and hit him with an aerial for a quick kill. Also, dsmash->reverse tailspike works in this position for some DIs, and can lead to grab->bthrow for others. If the falco tries pillaring you, you can DI off the stage and use your lengthy recovery (as long as the dair spike stun doesn't last long enough to outright kill you) to return. From the edge, you can try to bait a dair/dsmash and then hop a uair from the edge. This will knock him a little up and behind you (off the stage), and you can occasionally gimp him with another uair or nair before he can recover. Once either of you are at higher percentages, staying near the middle of the stage is better so that you don't outright die from a well placed bair/nair/smash.
An exception to this is on any stage where you can chainthrow him (FD, FoD when the platforms are down, etc.), since you gain somewhat of an advantage if you are able to grab falco in the middle of the stage, as he can't DI towards the edge of the stage very quickly.