You can catch Mewtwo or go beat the E-4 again
you don't need a national pokedex to trade from ruby to leaf green. You have to get the gems the sapphire and the ruby in order to trade from leaf green to ruby. These are found in leaf green. (:
it is omastar
Yes.
You have to catch at least 60 Pokemon and you have to beat the elite four including your rival
its bulbasaur, and you need to migrate it from fire red or leaf green.
You can link and trade Pokemon with Sapphire and Emerald Versions. If you want a national pokedex, trade with leaf green version
after you beat the elite 4 you should automatically get it if not see professor oak
cant YOU CAN MIGRATE IT FROM POKEMON FIRE RED OR LEAF GREEN AND GO TO THE PAL PARK AFTER GETTING THE NATIONAL POKEDEX
You can't catch Charmander in Pokemon Diamond. You have to import it from Leaf Green or Fire Red.
Poliwrath
You have to have at least 60 different kinds of Pokemon on your pokedex then go talk to professor oak. your rival shall also appear and he will give both of you a national pokedex.
In order to acquire the National Dex in Pokémon Sapphire, you have to trade with Pokémon Fire Red or Pokémon Leaf Green.