My understanding is that Dresser does not concern itself with the index numbers within each clothing category and the numbers do not stay static anyway. Is it not the case that when a sim picks up Formal outfit Number 7 that it becomes Number 1 for them and the rest of them get renumbered accordingly or is that only the case when the player directs the sim to change outfits?
I don't use the Dresser mod myself, but it certainly has a loyal following. I think you may want to bring this question to us at NRaas for what might be a better answer.