I don't think a quarry is viable in an instable world. And I also think you are unable to get an age with dense ores that doesn't have either meteors, decay or erosion. All three will eventually eat up the entire world making your quarries moot.
And it doesn't really matter. If you need valuable resources fast (diamonds, gold ore, lapis, redstone, red power gems, etc.) and don't have GregTech (I don't know how it interacts here...), multiple dense ores are fine. Just make it a fire-and-forget world. You can get good enough mining equpiment easy enough to get several stacks of diamonds in less than 30 minutes in such a world. By that point, the decay probably got to the lower levels of the world and you can scrap it.
For automated mining, I would use a simple flat world biome with 64x64 quarries. But I am not sure this is optimal.
It may be better to use quarries on a cave world (on the top layer covering it). They *should* skip through the vacant space, only picking up the valuables. Quarries in a normal flat world pick up tons of cobblestone and dirt before they get to anything worthwile... Any thoughts on that? How could you further optimize quarries in a Mystcraft age and how the hell do you reliably power them?