Hi Gisnar,
I too would like to know what you are using for 8mm scanning.
Could you tell us?
You're right. Since you're scanning film, that's different.
Targa was made for broadcast interlaced video, it's not so good for film (film is different, has no interlace and is better quality than broadcast tape).
Given the choice between JPG, PNG or TIF, JPEG is lossy (even when zero compression is applied) and degrades image quality.
If you want image quality, use either PNG or TIF (TIF is mostly for archiving or print-design, very high resolution work, and produces massive files which isn't what you want for video), PNG is good for loss-less compression and ideal for scanning film/negatives, you could also use it for archiving, but it's less popular if you want to print from them. PNG is also very good for video!