We've covered this match-up before in the Fox forums, so here's some basic, recycled information you can use:
-Uthrow Uair does not combo at any percent on Peach. However, I'm sure you know that because of the horrendous speed on her double jump, she is almost helpless once thrown in the air. Her only move that hits below her (besides strangely angled bairs) is dair. But we all know what will happen if Peach decides to take that path and combat your uair with her dair. Bottom line is, even though uair isn't 100% guaranteed ever - it still works alot of the time because she can't really avoid it.
-Peach is floaty and dies early on many of the low ceiling levels. Know when to start going for drillshine usmash kills.
-Peach has the option of relying on crouch cancels to secure some good damage against Fox. Watching for blatant crouches and other CC baits can lead to free grabs or dairs.
-Listen to Unknown, if the Peach doesn't know how to retreat with the float, try to nair her right after she's done a move. If you see the start-up of fair, don't challenge it. If you think fair is all the Peach is going for, then nairing first would be good, because of fair's start-up frames.
-On edge-guard the parasol is disjointed and fast. Don't bair unless she's high above you. If you think uair would kill her on edge-guard, go for it as long as you'd land on the stage afterwards. If she's floating level or a bit higher above your body and you think you can reach her with shine, go for it. Bairing her sends her super high can potentially makes things more difficult. It provides her with multiple methods of recovery.
-Peach is completely devoid of a ledge-hop attack because of her double jump. Her get-up attack has a very specific range, so if you can remember it and adapt to it - you'll be safe from virtually anything she attemps after she grabs the edge. This makes it very easy to lay down a solid pressure game.