I really wouldn't suggest Cass right now. She's still very much a late game powerhouse, but getting to that point isn't very easy with her recent rework. Stashu is definitely trying, but he's a little lost at the moment.
Chamions like Fizz and Zed are actually quite easy to shut down if you're careful about it. With Fizz, try and find out if he's aggressive, trying to get early kills, or passive, and is more focused on farming and racing you to level 6. If he's the former, punish him by predicting his Es and Qs, make him waste mana, and let him push your wave until you're farming under your turret and he can't dive you without risking being stuck in the CC you should have against a Fizz, or making himself open to ganks from your jungler without the mana to get away. His trollpoll is VERY expensive and he can only Q through one of you to get distance. If he's the latter sort, PERFECT! It means that your own laning phase should be relatively safe. As he starts getting closer to six, start suggesting to your jungler that he stay nearby just in case. Save your mobility options and your CC for when he starts diving. Don't blow anything until he's using his trollpoll, as that's his main method of getting into and out of fights. After the laning phase is over, if he has a Lich Bane do NOT 1v1 him. That's exactly what he wants. Keep your distance, try and keep vision, and make sure you're warning your laners, usually bottom lane, ANY time he's not on your screen. He's a slippery little bastard and he loves to R E Q into a bottom lane trade to pick up a double kill
With Zed, his Qs are actually ridiculously easy to juke. If you see that he's being heavily aggressive, stay on the move. Don't move in any patterned way, and watch him more than you watch yourself. The wind up animation for his Q is pretty distinctive. When you see him cast his shadow, stay very far away from it. It'll really hurt early on. Zed is a very position heavy champion, and he's going to try and force you from lane by building a vamp scepter or taking life steal runes more than likely, so focus more on survival than necessarily winning. If he can't force you from lane he also can't roam. Building a Zhonya's Hourglass as a first item can be incredibly potent, as it eliminates the bonus damage he accumulates from his ult, and if he hits you with his ult try and Zhonya's under your turret so when he shadows onto you he's forced to take at least one shot... several if you hit him with CC while he's under it. Assassins might seem hard to deal with, and that's because they are... if you aren't cautious. They're built to punish players for making small mistakes in their positioning or skill usage. Turn it on them and instead force them to make mistakes of their own, and they lose much of their ability to get out of sticky situations.
I'm not actually online right now, it was a hell of a day at work and I'm not too much for playing when my back is giving me issues (sucks to be getting old at 22), but I'm glad I was of some help.
And yeah, I can see how that'd be a wombo combo. Lotta knockups, stuns, snares, and roots there. Malphite/Yasuo is bad enough as it is.
Also, just as a side tip, try and avoid Yasuo. He suffers from the same issues that Katarina and Yi do. A lot of damage output, very high mobility to dodge things, but very easy to shut down... even if he's a bit easier to keep that from happening than either of the other two. I actually have a pretty strong bias against Yasuo's and Yi's lol. It might sound salty but I'll usually auto report either that I see for being unskilled after a match is over purely from picking champions like those.