What kind of settings should I use when I use images that are scanned from the negatives of photo's? All photo's are taken with the same camera, but due to the scanning the pixelsize of the photo's differ.
Should I define 1 camera for all images, or a new one for every photo?

If the pixel size for each image is different you have two options:
 
1. You must allocate a new camera for each image
 
2. If they were scanned in from the negs then you should be able to crop them to the black neg frame and they should be the exact size - in which case only one camera is used.
 
TIP:  if some images are landscape and some portrait they will need new camera too!