
Marth can be troublesome if the person plays very grabby.
Grabby Marths (I mean more grabby than usual a.k.a. grab spamming like laser spam) will make this matchup extremely difficult. The Marth matchup isn't a bait and punish kinda deal, though, although it seems like that at first. The idea is similar to how characters try to beat Meta Knight in Brawl; you can't really bait because both characters are fast enough to react to different situations, but you are generally supposed to take advantage of the committal nature of their moves; that is to say, once they've started a move, they can't be very flexible with how they're moving or using it (this applies more to aerials and sliding moves) because of the duration of the moves and often the specificity of the type of attack used (this applies to grounded moves but can also apply to aerials).
So it works like this: typical bait-and-punish is what we saw a lot of in 05. You dash dance to force forward smashes and spotdodges or whatever (with whichever character). Play has become so quick now, however, that simply dash dancing (even very MENACING dash dancing, LOL) just doesn't cut it anymore and players can react more quickly with more precise moves with a higher success rate these days. That being said, simple bait and punish doesn't work. Now, the idea is not to force a move out of Marth, but rather to get him to swing at you naturally with the opponent under a fair degree of conviction that he will either 1] hit you or 2] wall you. In either instance, you want one of two things to happen: 1] don't get hit or 2] tank the hit and hit back harder.
In the first case, which is the most ideal for the short term, avoiding damage is obvious, but the key is finding a way to get in range quickly enough to make a hit that counts. For instance, let's say you manage to spot a forward smash from a low percent Marth coming at you while you're not being comboed (for whatever reason). Now, you can wavedash away, roll back, spot dodge, triangle jump, etc. to try to really get in range enough to punish this move. Your own forward smash counter (which is one of our tried and true techniques) isn't really the BEST option, per se, since Marth is at a low percent and using the forward smash now will only lead to some damage and weaken the move damage-wise for the next thirty seconds of battle or so, making it a less favorable option later on in the stock. Did you know (and here's some helpful Yoshi math--I love number crunching in this game) that if you manage to spot a forward smash in time on a Marth that is around or at least at 10%, that the best way to punish is to roll INTO him? When you roll in, you can up tilt out of the roll and, IF YOU FOLLOW DI PROPERLY, you can score a triple DJC up air into an aerial finisher and it's pretty much guaranteed. How much damage is that? More than forward smash, which would otherwise be the go-to counter in a similar situation with a few differing parameters. This first case of avoiding getting hit works in the short term because it leads to heavy momentum swings and lots of burst damage.
In the second case, I'm mainly referring to DJing through aerials to counter. You might be saying to yourself, "That sounds like suicide at the very least, especially against such a good character," but one of Yoshi's greater general strengths that extend across many of his matchups is that he can go blow for blow against most characters in the game and still come out on top because of his high overall survivability. Pound for pound, few (and I mean this) characters can match his knockback and damage output while still being able to survive on the other end when you constantly slam Yoshi and another character at each other under a reasonable set of circumstances. If you're being comboed, try to eat a non-tip move and jump into the next one AS YOU SEE IT COMING. This doesn't really work if you just jump after you get hit by something expecting a follow-up hit because then if the other person messes up (F*CK BAD PLAYERS THIS HAPPENS MORE OFTEN THAN YOU THINK) or just decides to bait YOU (!!) then you're worse off than Sheik using her double jump. So wait and see what the next move is, jump through it IF YOU HAVE A GOOD FOLLOW UP FOR WHAT YOU CAN COUNTER WITH, and then counter and proceed to hit as hard as you can. This second case of tanking moves is good for the long run because while the first case is more favorable for the short term, it makes players more reserved to begin attack sequences over the extended course of a match, making them more campy, making you more lose-y. This case makes players more reluctant to attack so much DURING their sequences, making combos riskier, making them hold back on a front where most characters typically have no defense, and giving you (my favorite) a mental foothold.
In short, you want Marth to attack you. The more he attacks, the better off you are. The more you can convince him to attack even though you're somehow still hitting him, THE MO BETTA. It's a delicate balance between beating him up and giving him enough damage to make him keep wanting to stab at you.
Fox you can combo, but he should be tirelessly running away and shooting lasers at every single character in the game BECAUSE HE IS A HUGE F*CKING F@GGOT, so I mean, I guess that argument is kinda (?) moot, but I don't want it to seem like your opinion doesn't matter so I'll say, "Okay," even though I genuinely do not agree that Fox is winnable at all, much less anywhere comparable to even the hardest Marth.