It depends how good you both are with each character
Peach and Sonic is an interesting team. Both have very good recovery which means if one is in trouble off stage, either can help the other out normally. Peach's Floating means she isn't going to interfere with Sonic's ground game as much as she can just stay in the air and use aerials if need be. Both have decent damage racking abilities (Peach especially with her Dair) and Sonic's good grab game can be used to send opponents into the air for Peach (provided you get the chance and don't accidently throw someone into Peach). You can grab people and Peach can do ground floating aerials to hit them etc. Make sure you space properly though
However, both have difficulty KOing (Sonic definitly more than Peach). They both have a good offstage game but you may find yourself having KO issues now and then
I think they'd make a decent team provided you work well together and don't get in each others way. Having said that, if EMZ doesn't main Peach, then I would reccomend he uses the character who he's best with
Get some team practice in (vs CPUs if you must) and see how they work together