your copier doesn’t have the invert option SilverStag? my scanner with do both slides and negs and if i end up using old scanned negs plan on doing basically what you want to do.
the rule for photoshop generally is whatever you do must be done over the entire image. no spot treatment. see monkeybort‘s post above.
so no it doesn’t break any rules. have fun. we’ll look for your entry(ies).