Sheik-Fox is even. Fox-Marth is also even, perhaps slightly on Marth's side when you're on FD (because of **** combos/chaingrabbing). Anyway:
vs Marth:
Technical perfection is a must. Missing any L cancels will get you sheild grabbed, and when that happens (if you're up against a good player), quite often your stock is gone. If not, you get a lot of damage. First off, here are some tips on how to escape Marth's grab combos.
The most important thing to understand is that they're not always escapable. Mix up your DI. Try not DI-ing and shining them out of it (27-28% or above 30%, up to ~40%(maybe higher; I'm not sure)). However, Marth has an answer to everything you do. If you DI so that you go straight up (setting yourself up for a shine), you're also setting yourself up to get pivot grabbed, u-tilted or even tippered. If you DI forward, regrabbed or tippered. It's hard to say. Just be very conscious of what you're doing and never have patterns. Just DI randomly and do your best to escape. All you can hope for is that they miss something.
That might sound scary, but it's not like you can't do anything to Marth. You've got lasers. When you spam these, this amazing thing happens. Marth actually will come to you. It's a hell of a lot easier to deal with Marth when you don't have to approach sometimes. Not to mention it's extra damage. I can't really tell you any combos; I'm sure you know lots. U-throw to u-air is pretty **** though. Also, you could try being creative with the shine. Sheild pressuring is really hard, because (as I mentioned earlier), a single missed L cancel can lose you a stock. A lot of Foxes tend to run up and Nair to shine, and pressure like that. Nair can be crouch cancelled, and Dair is just a better options in any case. I like to shine > SHFFL uair to shield pressure. Usually I jump over to the other side so they can't sheildgrab me as easily. If you get a shine, you can get a grab. This matchup is all about grabs. Grabs can do hideous things to eachother.
As far as combos go (I know I said I wouldn't talk about this, but I changed my mind), u-throw to uair is good, but chaining them can get you punished. I like to do that just for damage but above 40% or so I like to bair. This way I can land more hits. Fox is fast, so what I do is try to lure out an aerial and then get another bair in. I use this to get them off the edge, and I just space bair's for edgeguarding. It's one of the best options. About luring out their aerials, almost all marths will save their double jump until they get near the stage, so they can double jump fair and attempt to make you roll, so they get the edge. A simple solution to this is to fast fall down and come up with a bair right after their fair. If they start getting wise to your tricks, then well...what could they really do, save their aerial and sacrifice being able to grab the edge? Not that this is the end all tactic, but it's very effective in most cases.
Just be smart against them. Don't approach when you don't need to, and study yourself to find out what you're getting punished for. Marth punished like there's no tomorrow. Mew2King, for example, tends to take a stock off every time he grabs someone. A lot of good Marth players are like this, so just be careful.
Possible counterpicks against Marth: I pick Poke Floats. Actually I do that to just about everyone, but there's a lot of camping space here, various walls to drillshine on, and lots of low cielings. Ban FD. Not only do most Marth players like big spaces, they like FD because no platforms = no escape from their gay combos. It's all a matter of preference though. I often pick Yoshi's because I can control the space well. Same with Battlefield. Sometimes I go Rainbow Cruise and Brinstar. Just try stuff out. When you recover, do so only to recover and not to deal damage. That will get you killed unless your opponent messes up or just sucks.
As an alternate to everything I just said: go Sheik.
vs Sheik:
This is a fun matchup. Shine > grab > u-throw > u-air ***** Sheik. One of the biggest things to keep in mind is that Sheik has a crazy tech chase game. Play with the assumption that Sheik will do these things based on your DI and teching:
- she will follow your DI with a wavedash
- she will chase your tech by running and grabbing you
- if you tech in place she will down smash
- if you don't tech she will downsmash
I actually don't know as much about this matchup, because I don't usually play it. Most people only bring Sheik's out against my Marth (go figure). Sheik can gimp Fox really easily. My Sheik plays on the edge vs spacies, because if I get a grab on them I can usually kill them. Recovery against sheik is hard. Her bair and fair beat your firefox. Her needles will **** it before you even start. Try to make sure you're not at a good angle for Sheik to needle you, because you'll regret it otherwise.
Camping, once again, is important here. Sheik will try to camp you too, but in this matchup lasers are better. If she has fully charged needles, just pressure her with close range. Those needles will get a lot of damage on you if you let them hit you, so don't let her throw them. This isn't easy. You'll just need to try to get as many grabs as possible, and make good use of them. Uair ***** Sheik, as I already said. If you get a shine, you can u-smash too. Fox has got options here.
Finally, on edgeguarding Sheik. This can be really hard because of the invincibility frames/the explosion (which if it hits you can give Sheik a free kill via edgeguarding (I do it all the time to spacies that screw up)). If you don't want to go out and just hit her before she uses up-B, it's a guessing game. You can trick her. Basically, sheik has two options: grab the edge or land on the stage. You have to make her guess which one is safe, then punish her for it. Try to lure her into grabbing the edge, then edgehog. How you choose to do that is really up to you.
Hope this helps.