Well you won't be able to have the Platy's in a 5 gallon I'm afraid. Not to sure about the Guppy's 3 might be ok. But snails and shrimp would be ok, as long as you got a sensible amount.
Breeders will figure an adult mystery snail at 2 gallons each. If you want guppies as well, you might get away with 3 male guppies and a single mystery snail. At that point your tank will be full but won't look very full. Shrimp are often figured at several to the gallon so a few shrimp could also be added without too much overload.