At the moment if I work on a file (make adjustments on a jpeg) in a project in iphoto can it be read when I open the file/album in Aperture? Or vice versa.
That depends on your Aperture version. Aperture 3.3 and later and iPhoto 9.3 and later share an unified library format and can work with each other's libraries. You can edit in iPhoto and use see the edits in Aperture, see: Aperture 3.3: Using a unified photo library with iPhoto and Aperture
Also is it easier to work in RAW files in Aperture or iPhoto?
You can work in both applications with raw images,but Aperture has the more versatile controls to adjust raw images. You will gain more from shooting in raw, when you have Apertures adjustment tools.
If I copy these files to a hard drive can I remove them then from the laptop safely and will my projects and albums holding using these files loose the images.
Aperture can reference images on external drives. If you are running out of space on your system drive, you can select projects in Aperture and let Aperture relocate the original image files to an external disk. All your projects and albums will still be able to reference the image versions with the external, referenced original files, as long as your external drive is connected. See this section in the manual: Aperture 3 User Manual: Working with Referenced Images