Peach/Sheik play out pretty similarly in the grand scheme of things imo, mechanically you should differentiate between the two different dash-attacks/spacing options these two have you so can accurately bait options with your dash-dance. Otherwise, control neutral with dash-dance, remember Marth's threat of sword is more dangerous than his actual swings, but do throw out a dtilt/fair if you catch them slipping on their spacing. Grab -> uthrow on both, and focus on making it as difficult as possible for either character to come down safely.
For Falco, most Marths I notice struggle fundamentally with lasers, then get smacked around by everything else because they're so confused. Powershielding is a good option, another one is simply learning to deal with laser stun because it's actually deceptively low stun, and you can usually dash out before Falco can get any followup off his laser. Make sure you have your punish game down against Falco (both chaingrabs and gimp setups), because Falco will probably win neutral a bit more, you just have to convert harder on your openings.
Tai I've noticed likes PS -> wavedash forward into ftilt/fsmash. I like tank laser -> dash dance grab if they get predictable with laser -> nair/dair. Also, be aware if a Falco is starting to just sit still and standing laser, and try to snipe him with WD forward->fsmash because he's actually pretty vulnerable in the middle of his SHL before the laser comes out.