Spacie match ups dealing with like nair shine, etc is really match up specific. If its fox let him do one repitition of nair shine, when he goes for the nair after just nair oos and float toward or back depending if you hit them.
Kels plays very patient and well... Is really good lol so youre gonna probably have to play for a bit to beat him xD. But in ideal you are supposed to use turnips and use them to block his jump options. Throw it above him or onto a platform to trap him and pressure shields with nair/fair. I make it sound easier than it really is, you're probably gonna have to practice it a lot, just remember that peach hits hard but slow, so you have to work on trapping people.
Peach v falcon is honestly just, a really really bad match up for her lol. You kinda have to take a lot of risks which is unfortunate. Pull turnips if you can put enough space between you and use them to try to force falcon in his shield and just pressure.
Also don't JC grab if you do lol, her dash grab is a lot better in almost every way.