Yes, you can trade Ruby/Sapphire Pokemon to Colosseum, and use those as well to beat the Mount Battle. But most importantly, you have to beat Mount Battle in Colosseum Mode, not in Story Mode. The trainer line up is very different, and they are all at the same level as your highest Pokemon (so if your highest Pokemon was L70, all the opponents would have L70 Pokemon). But they will be at least L50, so you need to bear that in mind. And they will be using legendaries and rare Pokemon, so be warned. Only two real fights caused me problems in doing this, number 80, and number 100.
Spoiler: Number 80 has Regice, Regirock and Registeel among his Pokemon.
Number 100 has Kyogre. Be warned.