It was the example you chose so it must have felt the most aggravating to you. Let her wear the skirt. She’ll learn.
If kids are “strong willed” (and notice this label is almost exclusively given to girls 🤔) it’s because there are too many rules. Instead of thinking of whether she is doing what she is told, reframe it as, is she doing what’s appropriate? Are the rules important? Does it matter if she wears the skirt? Does it matter if her room is tidy? It’s her room. We have a rule that you keep communal spaces tidy, but your room is your room and as long as it isn’t a health hazard, the door can be closed on the mess if I don’t want to see it. I have never once insisted my daughter tidies her room. As a teenager she decides sometimes she needs to do it and sometimes it is a complete mess. If you take away the little rules she sees no sense in, she is more likely to follow the big ones which are important.
As for your other children, the one thing my mum did which has messed all of us up for life, was insist we all had to be treated the same. It wasn’t fair, it was the same. My two siblings and I are such different people that trying to have us behave the same way, follow the same rules was ridiculous. I was the smart one so sitting down each night to do homework was easy for me. My sister was not, and for her those times were hell. We were forced to eat the same food for breakfast every morning whether we were hungry or not.
Have a house meeting and draw up a set of half a dozen house rules, let the kids have input. Chat with your children as a group about how you are all different people and need different incentives. Focus on rewarding the correct behaviour rather than punishing the wrong behaviour.
Do not, under any circumstances, try to destroy the strong will your daughter has. She will grow up to be bold and fearless and strong and that will serve her well.