You can't catch Darkrai in Pokemon FireRed, it is a 4th generation Pokemon so therefore unavailable in any of the GBA Pokemon games.
None of the 4th gen Pokemon exist in any of the 1-3 gen Pokemon games meaning nothing like electivire exists in firered or any game before that.
Ch'ding is the nickname for the Farfetch'd you get in the game, which does not evolve. It does not have an evolution in any Pokemon games, not just FireRed.
Riolu and other 4th generation Pokemon don't exist in previous generation Pokemon games this means it can't be obtained in firered or any game before that.
You can not get Drapion in FireRed. It's a Generation IV Pokemon therefor it doesn't exist in any games before Diamond and Pearl.
Mewtree is just a fan-made Pokémon and it does not exist in any games.
You can't get mew3 in Pokemon FireRed. It is a fan-created hack, and can't be obtained in any of the legitimate games.
Well, you plug it in like any cable cord, but it only work for games connecting to Pokemon Firered and Leafgreen. As in, Pokemon Ruby, Sapphire, Emerald, Firered and Leafgreen.
You cannot be a gym leader yourself in any of the Pokemon games, so far.
Gyarados doesn't any further on Pokemon FireRed.
Pokemon FireRed, Legend of Zelda: Minish Cap, just to name a few.
You have to trade it in from any other Pokemon games like Pokemon FireRed or LeafGreen and transfer it from there. Or find a friend who has Charmander and trade with your friend.