Against Puff I just walk around trying my best to get under him while he's trying to wall me, but all I can get If I get there is one hit, two hits tops and then I get ****ed.
And it's like impossible to edgeguard properly.
Welcome to the Puff MU. This MU emphasizes a ton of movement techniques because ground & platform mobility are two of your biggest strengths over Puff. Shield > WD oos is therefore invaluable in this one. Being really good at movement around the stage is important so you don't get cornered. Dash > WD, WD > dash, and wavelanding are all really important techniques to have at your disposal.
I'm not really sure what kind of help you're looking for because, yeah, you can't really combo Puff that much. I do have a few tips to help your damage output though.
At low percents: I like to run around the platforms from 0-20ish because if you can bait her into attacking into your needle spray area (Puff needs to be airborne) when you're on a platform then you can do aerial needles > run off fair > another move (sometimes f-tilt, sometimes nair, sometimes something else entirely - it depends) at low percents and sometimes get a decent air combo going but that's about it for heavy punishments.
A good general piece of advice is to keep needles stored at all times because they open up options like needles > d-smash, needles > d-tilt, etc. A generally good, underutilized tactic is to combo your stray hits into needles. Throwing your needles at Puff after hitting a stray SH FF Fair or whatever can add an extra 10-18% to her, which is dandy. You'll likely have opportunities to charge needles in this MU because of Puff's defensive style (and because you will be batting her away periodically).
In a similar vein, against exceptionally defensive Puffs, if they space so far that they're landing without being close enough to hit you, it can be worthwhile to shoot needles at him as they're landing. You can rack up damage this way.
Chain is actually decent under platforms if used sparingly and you retract it quickly after hitting it.
At about 90% you can dash attack a crouching Puff and combo into up air for the KO because the crouch reduces the KB of the dash attack to enough that you can up air and KO. Dash attacking through Puff at like 50%+ is a reasonable play because it knocks her over and you can tech chase her. You have to space to land very far behind her, though, and this is still kind of risky. Use sparingly. Down smashing a crouching Puff is also a decent thing after like 40% to push her away (it's great if you can push her onto the edge).
Edgeguarding Puff is hard. Throw needles if you can. Ledgehop Nair stage spike them if possible because Puff players are notoriously bad at teching. Needles > fair offstage works often. This is all vs the low recovery. Against the high recovery just try to stay under her and see if you can bair her. Don't get baited by Pound's monster priority and wait it out. You really shouldn't get successful edgeguards on Puff very often, but on low recoveries it's feasible sometimes.
Against Peach, well, I hate Peach. I feel like I'm doing better on Peach than on Puff, but get ****ed/gimped/whatever leads to death way more at the same time -_-
Plus, I'm way better against Peach with Falco than with Sheik, as I can take some space control and lead the pace of the game more easily, but I feel like I should be more confortable with Sheik...
Peach is just footsies and there's a lot of gobbledygook regarding the shield game because Peach's shield is good and her pressure is good.
For combat:
Fair, bair, aerial needles > grab combo, and grab are your main tools. F-tilt and d-tilt are also good. D-tilt is mainly for swiping under nairs and bairs. F-tilt beats or trades with things because it's f-tilt. Don't limit yourself to just these tools, but they're the main ones you should be using.
If she's on the ground, all of the above is good (except the tilts). Just be careful because she's likely going to try and dash > crouch to make your spacing bad on fairs so she can down smash. Or WD back d-tilt so she can ruin your spacing. Or dash attack under you. Being aware of these options will allow you to tweak your spacing accordingly. Generally I think bair (as a move), fair (as a move), and empty jump > wavelanding away from her ground game good actions if you're unsure what to do. You can also just maneuver the ground if you want and look for grabs vs her ground game (which is also a very good, very safe course of action). Her ground game is pretty vulnerable to shield play minus the low float aerials (her grab range sucks and if she misses you get a free move so keep that in mind and WD back OOS a lot to bait her into trying to grab it and other stuff like that).
If she's in the air, use your bair. If she's at a height where f-tilt works then do that. If you can fair her out of her float into f-tilts then that's recommended. If safety is your concern then bair is probably the best thing vs an airborne Peach.
In terms of combos on her:
0-20% CG if she DIs away (you must JC grab) - After 20% try to get F-tilt > Fair (not a true combo; you kind of space the Fair to beat her Nair). You could also do something gimmicky like FJ Fair > falling needle and see if you can get a regrab, which sometimes works. I've done SHFFL Nair > Fair before as well, but that's obviously DI dependent. SHFFL Nair (or Fair) both open up opportunities to continue pressuring. You can d-smash to follow sometimes (can beat combo-break Nairs), shoot needles, or simply apply pressure with movement (and look for Nairs, air-dodges, etc. you can dash dance grab). Plenty of options.
Otherwise up tilt > up airs / fairs (0%) and then follow from there (blocking their combo-break Nair and shield-grabbing it counts as a follow, but if they float away obviously this doesn't work). After like 20% just up air combo if they DI elsewhere.
To get grabs when you're on offense you basically want to pressure Peach into blocking (spaced aerials are excellent at this!) or doing a bad crouch down smash and then work from there (spaced aerials are also excellent at this!). You can also dash dance grab or whatever. Needles into grab is also good for punishing them being stationary.
To get grabs when you're on defense is tricky because her pressure is crazy good. That said, you can shield grab the jabs. You can also WD OOS grab her for d-smashing your shield (provided it doesn't pierce). However, there are certain conditions that affect your ability to do this and I don't wanna write it out. Instead, I'm going to suggest you WD OOS down (or forward VERY slightly) and wait for stuff like a 2nd d-smash, jabs, or sidestep that Peaches like to do after the down smash and punish that instead. If you're good out of shield, then by all means punish her immediately. But most aren't and it kind of depends on how many times she hits your shield. You can also jump OOS and try needles > grab if you have some stored.
I play Fox and Sheik both at high level (Sheik much more than Fox, but I have some experience with the fuzzball). Assuming you have the tech skill down for both characters, I think they're about equally difficult to actually play. Fox is harder to learn, though.