1) How do I set my iMac to default to pull my photos from the external hard drive, and
Sorry I have no idea what "pull" means in this context. Do you mean "Import"?
2) How do I default it to save to the external hard drive at the initial import
You can put your iPhoto Library on the External Drive. Simply make sure it's formatted Mac OS Extended (Journaled)
1. Quit iPhoto
2. Copy the iPhoto Library from your Pictures Folder to the External Disk.
3. Hold down the option (or alt) key while launching iPhoto. From the resulting menu select 'Choose Library' and navigate to the new location. From that point on this will be the default location of your library.
4. Test the library and when you're sure all is well, trash the one on your internal HD to free up space.
For iMovie, best to ask on the iMovie forum.
Regards
TD