What size tank is it?
But yeah, Skirts can be temperamental. Some are okay and some are just plain mean. I think if you have more of any shoaling species, they tend to be more focused on their own species versus the other fish in the tank. That goes for fish like Tiger Barbs, other dime-bodied Tetras, and danios. But, it's always limited on how big the tank is, I'm going to assume it's a 10g? If so, I would not recommend them in a tank smaller than a 20 long or 29. They do get to be pretty big.
I would recommend instead some Cherry Barbs (non nippy) or Rasboras of any kind. They stay smaller and use different levels in the tank than your platy do which makes for a nice peaceful tank!
As for the snail, depends on what kind. I'll assume you want a Mystery snail and it's a 10g tank, if so, it'd be okay, you'd just have to supplement the feeding for the snail. Platy's are voracious eaters and will eat everything and even take food from the snail so it might be difficult to feed properly. Supplement green algae will also be necessary for them too. Mystery snails are big poopers as are your platy so I wouldn't recommend it unless it's bigger than a 10g. Otherwise, you'll be cleaning out poop left and right!
Wilds: Wajok, Rutilans, Siamentorialis, Hendra
Leopards: Luna, Astrael, Dorian, Cullen, Gallifrey
African Fat Tails: Nox and Lux
Bearded Dragon: Stormfly