Ok, so I'm really late on these, and I'm sorry. Fits of laziness conquered me.
Teams: As Peach, one of your roles is to kind of act as a wall to keep the slower character out, while staying within range to help your teammate if necessary. You seemed to let yourself get separated a lot, and the match became 2 1v1s, which is NOT the same as a teams match. Whoever you team with, make sure you guys work on being synchronized. And don't let them divide you up.
Singles:
A lot of what you seemed to be doing were things from nerves, which I understand, but if you know Pyro at all, he's not that scary. ^^; In any case, that'll go away with the more tournament sets you play.
Against Marth, DI his throw combos down and away, to avoid multiple followups. You were getting fthrow > fthrow > tippered, which you can negate by DI'ing down + away. There was a point when you up B'd out the fthrow chain...that was probably him messing up, don't count on it to work.
Recovering low against Marth is a bad idea, don't ever do it (his dtilt gives you NO height to DI with, and you'll probably die, since it's impossible for her to sweetspot the ledge with her parasol). Mix up between high recovery and mid recovery.
You need to be able to wavedash/dash dance versus Marth (make sure you don't do it too much) to keep him honest, and stay mobile. Once you let Marth lock you down at all, you lose. You seem to move very stiffly. She's awkward to maneuver, yes, and nerves probably played a part. So make sure you keep your techskill fresh.
You already know about the rolling. Rolling in towards him can be a legitimate strat (ONLY as a mixup), but you were doing it too much. Consider your other options out of shield (namely, wavedashing), and mix them in so that you don't get stuck rolling all the time. Also, tech skill.
Try to snipe Marth with dash attacks when he jumps (or as he's landing). These can lead into uairs, and other cool combo moves. Makes sure you practice your combos on Marth (they're somewhat difficult to time/land, so you need to make sure you can do them).
Time your spotdodges, and mix them up so that Marth won't read them. Spot dodging is "good" against him, just do it intelligently. When you use turnips, don't think so much about approaching Marth, but manipulating him, controlling him with the turnips. Look at how he reacts to them, and adjust your play to that. For example, if he likes shielding turnips, you can go for grabs more after throwing turnips.
Toad is bad. Try to use it (at most) once a match. But even that is risky, because punishment is high if you miss.
Edge guarding Marth is tricky. If he recovers low, just grab the ledge, and punish his onstage lag, or use the Marth killer (roll towards the edge, and with your back to it, hold down Z, and angle the lightshield away and down). If he's going low, and your on the ledge, don't give up the ledge until the point where he has to upB, so you don't give him the ledge, while having time to punish him if he lands onstage. Recoveries higher than that, you can throw turnips out to catch, or sometimes go out with an aerial to get him.
And more tech skill.
For Fox: This MU is still foreign to me, but I can tell you that wavedash back > downsmash is your friend. If you can call an approach from him (short hop nair), use that to punish him. You need to stay VERY mobile, while simultaneously stopping Fox's momentum, with jabs/aerials/downsmashes. Make sure you can.
Idle floating will get you upsmashed. Don't stay in one place for any extended period of time.
Make sure you know how to get grabs on Fox. You MUST grab him. It instantly puts you at an advantage, stops his momentum...grabs are too good. Building from that, make sure you can chaingrab Fox. Go into training mode, place a CPU Fox on 30 or so percent, and set him to Evade. Then just chase him down, grab, uthrow, and see if you can chaingrab him to 100. Make sure it's consistent.
I feel like this is a lot of babbling, but I hope it helps you. If you have any questions about what I said (if something doesn't make sense, is different from something you know, etc) just ask. You can also visit our main thread with other questions:
http://www.smashboards.com/showthread.php?t=242179