Go under Peach quickly if she's floating near and forward (unless you're giving up stage position doing this), don't be afraid to shield grab super high fc aerials (even if the other Peach FFs, they're probably not hitting shield particularly close to the ground if she's floating from a height further from that ground than say, side platforms on Yoshi's), FC fair, work on jumping with turnips to properly aim, grab, dsmash moves than miss. You can also DD punish floats, or if you're less confident in your ability to punish or challenge a float, roll or wavedash in the direction that puts you farthest from where they'd land given their current horizontal momentum if they chose to fall right then. Pull turnips when Peach is floating from less threatening positions, fsmash against moves they'd try to space against your current position or deny advancement with.
Most turnip throws are ideally from a position where you're somewhat lower vertically than the other Peach, she doesn't approach by going higher in the air, and you can clip lower parts of float. Turnips can also cover your own approach with stuff that isn't FC fair. FC fair should usually be done from a higher position, and is usually best going forward as little as possible or necessary to hit or prevent a guaranteed punish (when fading back with it, it's also harder for the other Peach to just go under you). Turnip from above are better when the opponent is floating back or at farther distances. You want to aim somewhere from center to feet for where ever you are expecting them to be or where you think they can move at that given point. Nair to beat other turnips (mainly if they're approaching as they throw).
FC bair to dsmash OoS. Turnip and nair recovery, covering again from lower positions to limit options. Dsmash at the ledge can be surprisingly effective against other Peaches recovering from low if they don't tech.
Double jump is kind of like a single use aerial dash dance. Floats below fulljump height can be countered by jumping or floating just over depending on starting distance. If they come in you come down on them with fair (bair is also fine coming down). Expect them to try to escape if they didn't get hit, but to be safe, try to space it or make them respect some amount of pressure from the fair before trying to punish rolls or wavedashes from fairs that were very close to hitting. Try to base your actions on if you hit a shield, nothing, or the other Peach.
When the other Peach is in the air but can't really come down with fc-fair for whatever reason, fc nair works great. It also works really well for combos, but in the event of CC, fair if you can, or do something to avoid getting dsmashed (also dsmash the opponent if they CC).
If Peach is in the air but lacks horizontal momentum, or isn't starting a fair, and you're in dash attack distance, you can just do it.
Lots of areas where dd grab or fsmash would work, dtilt also would.
If Peach is floating super close horizontally, and not doing fair, just jumping up with nair or uair can will likely hit.
When retreating from an approach, you can do fc bair facing away from the opponent, and then come in with something or wait for a reaction (retreating FC bair is amazing).
More importantly than like, any of this, be smarter with defending decisions: avoid spotdodging, roll before the other Peach's attacks are even getting to the ground if you do, try to get grabs and just throw her away/offstage. When below, look out for FC fair, and try to pressure safely. Aerial turnip throw -> fc nair at lower height that would hit well with appropriate hitboxes is a pretty general approach that isn't just fc from higher->fair.
