Here's some more info about Pokémon Colosseum from The Magic Box

Nintendo announced Pokemon Colosseum for GameCube, the sequel to the popular battle game Pokemon Stadium. This time the game will feature new Pokemons from Ruby & Sapphire versions, and the game supports up to 4 players simultaneously, you can connect your GBAs to the system using the link cable. Pokemon Colosseum also allows 2 VS 2 battles like in Pokemon Ruby & Sapphire.
Click here for screenshots They are a lot more clear then the CoroCoro scans. But OH NO!! the graphics are still too polygonal for my taste. I was expecting more.

Well, at least the trainers are present. I had no idea how the old unobtainables showed up. Did the game include them?