i play on painful or tough difficulty with tech trading off, and the Korath become dominant in every game i play. the only way i can stop them is to try to get the second and third most powerful civs to goto war with them until i can build up enough to fight.
i wonder if the AI can't handle the Spore ships. i have a hard time defending against them. they are ridiculously powerful, and turn your planets into poisonous hellholes even if you get them back.