You seem to like the f-smash. You might want to try the f-tilt more, though. It's quicker and less laggy. Even though you got KOs with it, even an f-tilt would have killed Fox at 150% on Yoshi's Story. To get kills at lower percents, focus more on attacks that get Fox off the stage, even if they don't KO him directly (b-air, u-air, f-tilt, d-tilt, b-throw). From there, edge guard.
Good ways of edge guarding Fox include D-smash or f-tilt at the ledge. You can even short hop when it looks like he might use a Smash-B (Fox Illusion) and then put up a n-air shield for him to run into, which actually out-prioritizes it. If he's going to do a Up-B, just jump out and hit him with a B-air or U-air, then go back and hog the ledge.
Speaking of edge hogging, it's really handy even when he has enough recovery to go up and on top of the stage. This prevents him from sweetspotting, and you can also use your invincibility frames to drop and b-air him as he comes up with a Fire Fox.
I wonder why you paused the game and said "What?" when you got U-smashed from a ledge roll. Kirby has a long ledge roll, but lacks any decent amount of invincibility frames. I rarely use ledge rolls because the first thing people want to do is smash Kirby when he does it. It's sad, and yet silly especially when he does it at 100%+ and squishes up and down. LOL. That's pretty much the only time I'll use it so I can laugh at it (and then get smashed).
