Pikachu is easily the best pick you have, but if you want to play Samus it's not impossible to win, just more difficult. I'll talk Samus.
Learn Samus' sweetspot when grabbing the ledge. It is incredibly low (it looks like you won't make it). Also, Up-B with Samus is really good if you're close to the edge and think you can hit them with it.
Vs Mario:
Samus' jab, B-Air and' F-Air can be used to swipe away fireballs and sometimes tack on damage (especially Samus F-Air).
Stay above Mario's level - if he's on one of the DL side platforms, jump and stay in the air above the other side platform, etc. Approach from above or the side, and bait his D-Smash by double jumping when you get near and then fastfalling with an aerial: D-Air>B-Air or just a N-Air.
Charge your plasma ball when you can, that thing eats fireballs and is a veritable threat that'll make him shield reflexively. Also note: if you are sort of close to Mario and hit his shield with a fully charged plasma ball, you can run up and use your grab for a guaranteed throw-off. If he likes to roll backwards you might even catch his roll.
Vs Kirby
Stay above him and watch out for U-Tilts. U-Tilts are not an impenetrable wall, but they are very diffcult to deal with. B-Air is big and can take advantage of the gaps in U-Tilt spam. It's also good for edgeguarding.
idk i might add more later if i have time