If Falco is really predictable with the fsmash, you can try to use an attack that has its hitbox out for awhile, like nair or pound, starting right in front of where it will hit. But fsmash has pretty bad startup, and is actually strongest right behind Falco. If that is your main trouble with him, try to slow your approach a little and bait smashes; they usually aren't very effective in spacing a patient opponent.
And as for Marth, I used to main him. Most Marths will try to approach you if you don't run (or float) right at them. It's a terrible matchup for Jiggs, but Dancing Blade is punishable if you can get behind it, and Marth's dair is his weakest aerial, spacing-wise (as in uairs are effective against him), although it's obviously a powerful spike so you may not want to try juggling him offstage. Also, Dolphin Slash is a great move, especially out of shield, but doesn't have great range as far as recovery is concerned. Gimping him is a very good idea, but he has an easy time stage spiking you if spaced improperly. Marth is invincible on frames 1-5 of this move though, so you need to hit him before he tries to recover. Also, he can use the first swing of Dancing Blade to stall a bit in midair. Fair is a great move for Marth, but he is better onstage where he has fsmash and the ability to use all four hits of Dancing Blade. Try to get him offstage as much as possible, whether through aerials or throws, and avoid his spike.
