You must defeat the elite four and the Pokemon champion then catch all the Pokemon which is only possible with help from friends.
Yes you can trade leafgreen version Pokemon to another leafgreen version.
you need to trade bellsprout, vulpix, and sanshrew from leafgreen and you can get the rest in fire red.
there are 5 legendary Pokemon in leafgreen version
LeafGreen version does not have Growlithe.
its only available in Pokemon leafgreen version sorry
firered and leafgreen version
lavender town
You cannot catch aipom in leafgreen however you can in emerald or Pokemon colosseum.
yes you can trade pokemon leafgreen pokemon to pokemon emerald version same as pokemon fire.
Gligar is a flying and ground type of Pokemon. The only way to get this Pokemon in LeafGreen version is to trade for it.
It is not possible to catch a Sunkern in Pokemon LeafGreen version. The only way to get one is to trade for it.
lavender town.