As far as the pika match ups go, I'd have to say 8:2 against yoshi.
Wow, looks like we need to bring out the scientific research on this one. You gotta teach me how to get out of his Dsmash Socks. I can't get out if my life depended on it ;_;
For Luigi, I learn why I should keep my mouth shut sometimes. He can combo, and he can combo well O_o. I've been talking to big and a few others if Luigi is actually harder than Mario, but they say it's the opposite.
Marth.... Is pretty much like fighting Melee Marth, except his reduced range makes it kinda easier to get in, and his NeuB goes straight. Plus Yoshi has the tricks to help him out. He's still Marth though. You gotta pick your attacks carefully, or else you'll be beat. I say 3:7, or 3.5:6.5
MetaKnight is best if you play defencive. If you try to head into MK Straight on, you'll probably walk out as one unhappy Yoshi. If you let Him come to you, then you'll fair a much better chance. Pivot's play a major part of this battle, because they can all snag MetaKnight right out of his Specials, which then you can CG, or Release Usmash him. Don't ph33r the Tornado.
It's also one where you want to spend as much time on the ground as possible, where he's forced to use approaches which can be countered well. Only go after MK in the air if you see an opening. You can beat him sometimes with a Bair, but Uair is your best bet. Sliding Usmashes work quite well too if he's coming from the air. Tilts help alot too, due to the overall quickness of them. You want to avoid using any moves that have After Lag, or are just generally slow. The Release Grab Usmash Combo helps alot to kill MetaKnight, where it's much easier to land a pivot grab on him rather than trying to finish him with a Fsmash. Still, it still won't be a walk in the park.