The Aperture 1.5 package came with information on this integration - do you have the manual?
Is Aperture referencing them or do you physically have two copies of each image, if the latter applies them you can just delete the Aperture library or use the relocate and consolidate command to move them.
You do know that Aperture will never let you alter the originial image! So even if you make the most radical changes you can always go back to original.
No, if you dont want the pictures in aperture then deleting the library will not screw up the program. The next time you start Aperture it will just create a new and empty library in the location you told it to (the pictures folder by default).
But again, you can reference your iPhoto library form within Aperture without taking up more memory (well a little but insignificant amount more).
BTW why use iPhoto at all when you have aperture?
The photos take up more space in aperture because you probably have the creat preview command turned on. If you turn it off, tell be about the same.