Ribbing as a type of Shirring?

So I am making this garment,Columbine and from all of the comments I’ve read of people who’ve made this garment it tends to be to big. I’ve decreased my needle size so I am smaller then gauge, but it is still to big. So I am wondering if I could do ribbing as a form of Shirring? I don’t know why I couldn’t, I just dont know if it will do what I want it to do. I am wanting the top to have more of a negative ease around the waist. Also if it would work when would I start the ribbing? I’m about 8 rows past the joining of the underarms, about a quarter through the bust. Do I start it now or wait till I am further through the bust? Also, do I just start the ribbing all in the same row or do a /\ to increase the width of the ribbing? Or would it be better to do the ribbing on the sides so they are kind of hidden by the arms? Sorry for the rambly post, this is my first time trying to make a garment with any type of negative ease that doesn’t originally have one