@MasterGary: There are some vs. Peach tactics on the previous page, if you haven't read them yet. In general, against a peach, what you should remember is two things: she's floaty, so if you can, KO her up (dtilt, if you can). If that becomes predicatable, she dosn't recover very well from your spike, so put her off the stage and see if you can get an early kill by throwing her too low for her whimpy vertical recovery (get into an edgehog ASAP after the spike. She'll try and grab the edge with her parasol).
Secondly, your lasers pull her out of her float. She will probably start beeing very carful about how high she floats to avoid them. Be creative with your lasers, spreading them as erratically as you can at different heights to make her approach really difficult. It helps immensly to be able to do this while retreating. Make sure you can not only RSHL, but reverse lasers at relativly any height you want. Such mastery of lasers takes practice, for sure, but can save you against peach.
When you shine Peach (or start pop her up into the air with any method), she's floaty enough that she can escape at mid percents. Don't go for fancy combos. The fanciest I'd attempt is shine -> dair -> utilt. After one of those, just go for shine -> uair to build some damage, then go for the KO with one of the methods in the first paragraph.
As for Mario/Doc...well, I honestly don't have experience with them. I'm guessing you can out-spam their projectiles, right? Falco is a lot more mobile with his lasers then Doc with his pills, I'm guessing. As for comboing them, they are semi-floaties, so you can go for some complex chains if you want to try. You won't be able to tack on as much as you can against a fastfaller, but you can give them enough that your traditional KO moves should work (fsmash, bair and nair).
Like I said, thats all speculation. You should wait for a more experienced player answers. Where's Shiz?