well firered and leafgreen have SOME different Pokemon so you can get those Pokemon from leafgreen.
so here is the site that list the different Pokemon. just scroll down to the chart...
http://bulbapedia.bulbagarden.net/wiki/Pok%C3%A9mon_FireRed_and_LeafGreen_Versions
It must be traded to you from emerald or Pokemon colosseum.
Have it traded to you from a Firered/Leafgreen.
Shroomish is not available to be caught in the wild in Pokemon FireRed, it must be traded from Ruby or Sapphire.
Shellgon must be traded from Pokemon Ruby, Sapphire, or Emerald. None of the Pokemon introduced in these games are in FireRed.
You can't. It must be traded from FireRed or LeafGreen.
You can't. It must be traded from FireRed of LeafGreen.
Sandshrew is not available for capture in Firered only a Leafgreen is able to obtain it so it must be traded.
You get one Johto starter in Emerald for completing the Hoenn Pokedex. You can trade them into FireRed. Unfortunately, Croconaws (along with Totodiles) cannot be caught in Pokemon FireRed. You must have one traded to you from Emerald.
Skitty as well as other hoenn Pokemon cannot be found in firered all hoenn Pokemon must be traded to you from ruby, sapphire, emerald, Pokemon colosseum and Pokemon xd gale of darkness.
Smeargle is a Normal type Pokemon known especially for its unique move Sketch. It cannot be caught anywhere in the wild in Pokemon FireRed and must be traded over from another game.
U Cant.. But U can have hoen Pokemons Traded from your game
Scyther does not evolve by leveling up in firered, It evolves by being traded while holding the metal coat.