peach is probably falco's hardest matchup imo. samus can do really well but peaches are just more consistent.
stuff i know, though:
dont dair at low percents. it can be crouch cancelled to grab or dsmash
respect her dash attack
her approach sucks, full hopping if they approach usually works well against the peaches i play with cuz they either eat a dair or they retreat.
peaches i know love to take defensive returns to the stage, so try to bait an airdodge or neutral b
spaced bairs work quite well
their edgeguarding can be predictable, as usually they can only cover single options and not multiple ones
i don't think i'd say peach is falco's hardest matchup at all. i mean, you have to play a bit different against peach but it's definitely in falco's favor.
as for not dairing at low percents wouldn't that only really apply to early dairs? i wouldn't not do a move as good as that (though i may try spacing it better)
the whole approach thing, i dunno man. if you aren't lasering her she can definitely get off on the whole approaching thing with that floatcancelling stuff and dash attacking
on the whole edgeguarding subject: how many options do you really need to cover against falco? lol if he isn't in range to jump right back onto the map then all he can do is try to side b or upb. side b only goes 1 direction and upb takes long enough for peach to get in position to edgeguard.
once again though, i probably don't know what i'm talking about so grains of salt for everyone