Yup, practicing in Melee as preparation for P:M should work out smoothly. In fact, depending on how far you wish to push your improvement, you should always be practicing in Melee to some extent if you wish to keep up with both games; Brawl physics carries over into P:M, which among other changes makes P:M much more forgiving than Melee in most aspects of the game (floatier physics, improved recoveries, etc.).
Concerning character-specific changes, the Melee top tiers (including Falco and Sheik) all get away with very few changes in the Melee --> P:M transition. As far as I know, Falco is a complete copy of his Melee self (excepting his silly new jab combo and blue lasers, huhu). I have seen complaints on the Falco boards about him feeling 'slower' or having 'shorter recoveries', but according to those who know what they're talking about, it's all in our heads - the only actual difference is that the character models from Brawl appear slightly larger than those in Melee. Of course, P:M physics may pay a slight part in it as well, but that shouldn't add up to much unless you're a nutty frame-perfect tech freak lol.
Sheik's main differences are the directions her throws send her opponents, which are (partially/fully?) borrowed from the PAL version of Melee; her D-throw sends opponents more outwards, which kills her chain-throw potential against anyone other than Bowser, Spacies if you tech chase, and nubs that DI towards you and give you free extras. I'm not sure how much her other throws are changed up, but her B-throw now leads into F-airs more easily. I remember reading up somewhere that her Side-B chain is less effective, but hopefully you weren't relying on that too much lol. Perhapys most importantly, Down-B isn't the worst move in the game anymore. Whether you hit it accidentally or are forced to bring Zelda out for recovery, not being able to immediately change back isn't nearly as hurtful for you as in Melee; even if you don't choose to practice much with Zelda, she's been buffed and is quite capable of handling herself with her defensive moveset until you have enough time+space to transform back. On the other hand, if you are willing to invest some time into practicing her, you gain access to a whole extra moveset of tools that will often throw your opponent off and force them to adjust (the smash community is fairly practiced playing against the Sheik MU, but few have much experience against P:M Zeldas).
Hope you enjoy your P:M experience! Once you get your Brawl disc and whatnot up and running, make sure to drop by the respective P:M character discussion boards for your mains. You'll find plenty of info explaining any other changes from Melee --> P:M that I may have (most definitely) missed, along with other interesting tid-bits that could come in handy for your improvement.
