I think peach is a very good teammate in general, however I find her better if she plays a support role rather than straight up aggro mode, unless partnered with jiggs or another peach. Her turnips are good to throw above your partner's heads and good to pick up behind them and she's fairly slow, but can excell in defensive maneuvers and recovering, so she won't die easily if you can keep yourself away from ko hits. That 1 stock you save might be the very thing to win you the match.
She's also an excellent damage builder, being able to sneak in attacks there and there, dash attacks and dsmashes and float cancels and turnipses... edgeguarding others is also her forte if she's not distrupted too much, just throw a turnip above your partner's head and bam!, headshot! dead opponent. Peach'll covers the ground game easily when your partner covers the air with quick shffles, sword swings and whatnot. People are a lot more prone to get dsmashed on teams than in singles and other sort of tricks she can do when people are too busy to notice *cough stitchface cough*
One thing she doesn't manage well is 2vs1 situation against her though, aganst quick chars. Marth she might be able to make hit his partner, but fox, falcon, falco, sheik... don't get caught in such situation and exist asap. She works best if you're close enough to your partner, she can also deal massive damage in close quarters and in tandem with her partner. I actually once did a double dsmash combo with sheik on battlefield, my opponents came from both below and above the platform. The above one ended up with 60% and below one 100%, from both our dsmashes that cancelled each others out on the same platform, flying off stage. And that was right at the very beginning of the match. =)
Peach can very well do very nasty team combos with her attacks. Weak hits send opponetns bouncing back to another one and so on. Her upsmash, when charged, is excellent finisher so your partner can just throw random people into it (or hit people into it, marth fair into peach charged upsmash ftw) and then get a nice ko. Sweetspot upsmash ko from grabs should be used a lot more. Personally I've thought of a funny trick for teams; have your partner grab someone below you while you charge dsmash on a platform and have them upthrow or whatever throw that sends em high enough for the dsmash for some nice cc damage. =)
For teammates...
Sheik:
Sheik is very effective in a way that sheik can pressure peach's worst matchups, fox and marth, with speed and stuff and keep em busy, so peach is a lot more free to do her thing, aka do damage, actually stay alive, edgeguard, help and combo with teammate and distrupt. Basically if peach is too slow to take on something, let sheik do the job. Problem solved. Peach is also very effective saver of this certain teammate due to that up b and combos can get very ridiculous and womboish as shown by team UGS (armada & aniolas). I do think sheik is one of the best team partners for peach, due to sheik's abilities to manage peach's worst adversaries and other qualities.
Fox:
This team has ridiculous damage and ko output in both straight up ko'ing and edgeguarding. Fox ko's up while peach ko's horizonal (and up as well, sweetspot upsmash omg), fox can go very aggro while peach can go very defensive, both can camp very well, peach can aid fox's recovery while fox can save peach from getting guarded easily. Fox manages marth and sheik while peach goes to demolish that falco. Imo, this team controls all the stages so well not many team top this one. Fox could very well go berserk on everyone while peach is just being a stock tank and deals lots of damage so fox and just go an upsmash people away when he wishes. Their double edgeguard is also very scary prospect, peach being able to cover from afar and above with turnips and float and fox shinespiking and bairing people from the ledge. If one player is very aggro, fox + peach is the team combo.
Fair to dsmash is also teh good.
