As my predecessor said seat tracks are fixed part of the floor structure. IT depends of which version of a/c you are talking about because it might have been that it was sold the other way around.
By adding additional (unused) seat track you just increase the weight/labour for no apparent reason, unless the airline decided to switch tem often. Anyway if you think a little bit the aisle would then probably have the seat track (unused one) right in the way so I do not think that was an option.
As for certifying seat track only the latest a/c have 16g since 16g seats are installed in the last 7+ years.
MD80 has 9g seats and therefore 9g sized seat tracks.
16G is requirement that got in from the military (plus all the crew seats are 16G).
Cheers,