in wich version, naw you don't have to say that if you are at the end of al games you can trade every pokemon back and forth, if this are the games rub, sap, eme, fr, lg, col and xd
In order to trade between Firered and Colo/XD, you must first defeat Evice(for Colo) or Greevil(for XD). This opens the trade center in the basement of Phenac City's Pokemon Center. If you have Ruby or Sapphire you can trade at any point as long as you have the pokedex. For Firered and Leafgreen, you need to beat the pokemon league, obtain the Ruby and Sapphire in the Sevii Islands, and take them to Celio on One Island so that he can complete the network machine or whatever it's called. For Emerald, you have to beat the Pokemon League.