Do you have a picture of her? I love to see furry faces!
Generally, it is not considered ethical or acceptable to breed a rescue dog with no known background. I would say that goes double for a 6 year old female. There's too many unknown risk factors, especially with a girl that is not exactly a spring chicken. What if she had difficulties in past pregnancies, or has never had a litter before? 6 is a little old for a first litter. What if she comes from lines with bad hips or eyes or heart problems or .... who knows what? You could accidentally double up on these risk factors be choosing the wrong mate and end up with a whole litter of sick puppies on your hands!
Did she come with her name, or did you name her? It seems like the breeders I used to work with had a litter about six years ago with a pup named Daisy. Maybe they could give you some background on her, if it's the same dog.