You should have captured him. If by defeated you mean captured then I suggest you use him.
There is no Mew in FireRed or LeafGreen.
sadly,no
The Cerulean Cave
There no Mew in firered.
Mew two sould be at the end of the cave in cerulean city after you beat the elite 4.
Mewtwo, yes, if you have a FireRed or LeafGreen with Mewtwo.
You must capture Mew on Emerald and trade it to FireRed.
You can't get mew in firered or leafgreen.
no because there isn't a mew-three just mew and mew-two sorry
Use gameshark with a pokemodifier, or trade it from firered or leafgreen.
You can Continue training or visit the Sevii islands
Capture Entei/Raikou/Suicune.