Yoshi's actually a really interesting matchup for Marth. I don't think it's too stage dependent, though DL64 can be very difficult to edgeguard him on. FD is likely your best bet followed by Yoshi's Story/Stadium. Battlefield is probably slightly in Yoshi's favor, and FoD is good in a lot of ways but every Yoshi player shield drops extremely well, so you have to be good at baiting and punishing shield drops. I recommend standing below and away and charging a shield breaker, or just hitting him with a no-charge shield breaker. On DL64/BF they have the option of going high for recovering as well as camping the top platform in certain situations which can be annoying. But neither are really bad, so just go where you're most comfortable.
Ness, you can go wherever you're most comfortable again, but his combos are best on FD and Stadium generally. I am partial to smaller stages because he's insanely easy to edgeguard, with FoD being my favorite against him.
For Y.Link, avoid DL64 as much as possible, and Battlefield can be rough too. FD is actually awkward against him too, because you can't stand under a platform to avoid bombs. FoD is, once again, my personal favorite, but Stadium and Yoshi's are great choices as well.
As for matchups,
Against Ness, his big strength is his offense game. Defensively, he's awful. So a lot of the times, I like to just get in his face with fairs and nairs (though stay outside of grab range). If you ever get him off stage, he will almost always try to air dodge back on, so read it. If he has to up B, I like to just jump out and get hit by it for the gimp. If you play aggressively while still respecting his dash attack, forward air, and grab, you'll be fine.
Against Young Link, you just want to avoid big stages and play aggressive against him. He actually has a really solid matchup against Marth (as does every low tier it seems haha), but your combo game on him is nuts. Don't give him space to pull a bomb, watch out for his really good recovery, and make sure you capitalize on any grabs/tipper fairs/dash attacks you get on him.
Against Yoshi, you have to play around his double jump. Marth has a REALLY hard time gimping good Yoshis. Just poke at him until he's at a high enough % to hit with a nair, bair, or ftilt and knock him out of his double jump. Grab a lot, and either up throw and wait for the double jump or down/forward throw and tech chase. Defensively, he's strongest when he's on a platform because of shield drops, so just make sure you expect that and don't go swinging at his shield from below. And finally, don't fsmash too much. Yoshis love that because it's so easy to parry. It's tempting because of how low it can knock him out of his double jump, but it's not worth the risk, except when edgeguarding if you don't think he's going to air dodge.
Hope this helps