i had a longer post but my computed bsod'd in the middle of it =/
don't go for the edge as often vs. marth. he's too good at putting sword hitboxes there. and sheik has her needles to watch out for. otherwise it's relatively safe. but gradually floating back onstage is quite good too...actually you might wanna default to this one, assuming you have a general awareness of how many jumps you have left.
in general, yes, crouching after l-canceled aerials is pretty good. it works especially well on marth and falcon (and you can rest them if you bait the grab and are close enough, hehe). don't do this vs. fox or falco cause they can grab you while crouching anyway.
upthrow rest doesn't work on falcon at 0. or possibly ever, i dunno. not reliable. a better choice would be upthrow upair rest, but this also doesn't combo at 0, since the upair doesn't stun him long enough yet.