With Leaf Green and Fire red you can trade with Colosseum to get Pokemon early. Collosseum didn't seem to have the rules and even bi-passed the Leaf green and Fire red rules.
Actually, Colosseum does not bypass the FireRed and LeafGreen "rules" in the sense that it is programmed to check the flag marking the completion of Celio's event. However, it ignores any other flags such as Elite Four completion or acquistion of the National Dex. In FireRed and LeafGreen's case, this is not a problem seeing as Celio's event cannot be completed without those flags being marked. However, Ruby and Sapphire, and more so Emerald, do present a problem.Ruby and Sapphire were arguably designed to be able to receive any trade from any game as soon as the Hoenn Dex is obtained, with FireRed and LeafGreen being the only games able to activate the National Dex. On the other hand, Emerald is more similar to FireRed and LeafGreen, in that the National Dex is obtained by means of an in-game event; trades of Pokémon outside the Hoenn Dex (or eggs thereof) are inadmissible unless the National Dex has been obtained.
Since Colosseum was not designed with Emerald in mind, Game Freak had to decide whether to program Emerald so that it would be identified by Colosseum as a Ruby/Sapphire cartridge or a FireRed/LeafGreen one. The decision was swayed in favor of the Ruby/Sapphire format for a rather simple reason - the flag of Celio's event was unlike any flag in Emerald, specifically the National Dex flag that is marked through an event triggered by the marking of the Elite Four flag. Consequently, Emerald accepts trades from Colosseum regardless of the position in the game or the species of the Pokémon to be traded.
Needless to say, Genius Sonority did design XD with Emerald in mind, for which the former game was programmed to check the Elite Four flag for Ruby, Sapphire and Emerald (the FireRed and LeafGreen condition remained the same).
But the above paragraphs do little to change the fact that as far as FireRed and LeafGreen are concerned, Colosseum does comply with their trade requirements. In other words, there is no loophole by which Johto or Hoenn Pokémon are made unable to evolve. (Even if there were, would it be remarkably different than Kanto Pokémon failing to evolve into their Johto forms?)
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