With DS2 the midwife asked me if I wanted to try one on his first night as he had worked himself up and was over tired. Took it no problems and spat it at about 4mths. He was breastfed till 10 mths so no confusion for him.
One gave milk, one didnt, no problem

With DD I took one in my hospital bag with me and used it the first night. She wasnt really fussed with the whole idea of a dummy though so we flagged it about a week later.
Again no confusion though as others have said it will get spat if they are hungry.

If you are worried about the whole confusion thing just try offering the dummy after you know she has been fed but just wanting the comfort.

Congratulations on your little girl by the way. Girls are great arent they??