I was told that soy is hard for people with cf to digest. I used the lactose free formula made by Natures One. It is the advertisement at the top of this page.
I have used it since my daughter was five months old. First we used the baby formula and now we use the PediaSmart. They also offer a discount to kids with gtubes. Good luck, I know we had such a hard time with Mariah and all of the formulas.