I like how you didn't even put jiggs as an option, like who would actually consider playing puff, and potentially puff dittoes haha.
I feel like secondaries are not easy to train and use effectively in competition, so don't be discouraged if yours don't work out at first. My biggest advice is : even if you do play a secondary in certain MUs, be familiar with the MU using Peach anyway, so you can fall back to your comfort pick at any time. And it's valuable experience, playing peach/puff taught me patience like NOTHING else ever could in this game. Oh I should mention I won with a time-out... twice.
So as Peach there are a few MU that are harder than the rest :
-fox. It's def. in their favor, but at least it's a very fun MU and it's not easy for them to not get CC destroyed.
-falco. Not too many falcos will be good vs floaties, but when they are it can get actually demoralizing.
-marth. I hate it. I quit this MU. sword too long. Can't do it w/out bombs or some ****.
-puff. Okay it's bad. The good side is that there are few good puff mains, and since there aren't that many peaches, you can expect them to be unfamiliar with the MU and not bair you 40 times up to the blast zone right away. But that's not worth very much if you don't know what to do either.
Which MUs do you like the least as peach ? That should limit your secondaries options.
-Both spacies are great in these 4 MUs but frankly pocket fox/falco doesn't exist (look at hfox lol). Too much of an investment if you plan on keeping peach as your main. I started playing falco and ended up maining him.
-marth is a good choice. It takes a particular finesse to do good especially vs puff. So you'll need to do some thinking and gain experience but it's worth it imo.
-sheik is easy to pick up and does very well against marth, but she's crap vs puff.
-falcon can work vs both marth and puff, is okay vs most foxes, but I'd opt to stay peach vs falco. If you have fun swagging around with him I'd say peach+falcon is a good combination overall.