You need the backfat because it has a different texture. Any other kind of fat will smear and the salami's will look grey-ish instead of dark red, but worse, they'll taste bad. They won't smell bad or give you an upset tummy, they'll be perfectly
safe to eat but they will not be
pleasant to eat.The meat and the fat need to stay sufficiently separate for a salami to work, otherwise it's wasted. So yes, stick to mincing a joint and adding back fat.
We learnt this through experience