Turn everything off w/ extensions manager, then when X starts up Classic it'll say something like "Classic requires missing resources, should I add them?" Say OK, then you'll have the absolute bare minimum required to run Classic. Now, just add extensions whenever you find a need for them on a case by case basis.
Because I always forget to change the startup set when I reboot into 9, I have two copies of 9, one for classic on my X partition and the "real" 9 on its own partition.