When photos on iOS are added to an album, does that duplicate the amount of space being used? If I add the photos to another app, can I delete them from the camera roll?
No, when photos from the
Camera Roll are added to an album, it does not result in double the storage space being used. The
Photos.app just tracks those photos as being part of that album, in addition to being in the
Camera Roll (which is the default album). Note that if you delete these photos from the
Camera Roll, they will get deleted from the album too (because there's actually only one copy of the photos on the device).
If you add photos from the
Camera Roll to another app and save, the other app receives its own copy of the photo. You can then delete the photos from the
Camera Roll and have them only in the other app.