I know what you are going through. It took us forever to figure out Max had a milk allergy. It was so inconsistent, but it finally got so severe, he would vomit everytime he had anything with milk in it, including my breastmilk if I ate anything with milk in it, and I mean even a tiny amount. And soy formula gave him diareha. We finally figured out he had these protein intolerances and moved him to nutramigen, but he still was throwing up. The milk protein is simplified in this which wasn't enough for him. So he finally went on neocate and then Peptamen Jr when he was about 14months. Thankfully, we tested soy at about 16 months and he was fine with it, and we just started milk, and he can tolerate it, although I am not giving him straight milk, just things made with milk.
Anyway, I was going to point out that if the preemie formula was bothering her, you can use any formula and make it 24 cal, 27 cal or 30 cal, depending on what you need. You just have to use a little math and mix more formula to get the extra calories. We do it all of the time!
DH, John (36)
Max, 1/20/03, 25 wks, 534g. (IUGR), severe PE