Mcgruff, congrats on the wee daughter. I have had 5 premies, and aren't they the cutest wee things !!!!
IMO when a child gets to about 6 months of age, to continue to grow at an optimal level, you must introduce solid foods, and this must form a major part of their diet. I am led to believe that up until about 6 months of age breast milk has all the goodies to sustain and nurture said babe, however beyond this point we need to introduce the nutrients that a child needs to survive optimally.
Personally I wouldn't be adding formula, I would continue with the breast milk, and also water.
If you boil up potatoes or kumara, coupled with carrots and peas - drain and then mash in a little butter and salt, add in a protein if you want....... this was always a great starting meal for my young uns. Also because a baby learns through putting things into its mouth, another really fabulous feed for her would be a cooked chop. They love all the fat and juices from the meat etc....
Anyway good luck, and make sure your wife enjoys that breast feeding quiet time, its all over too quickly !!!!
"never let the truth get in the way of a good story "