Teeny tiny infants do best with formula or breast milk or a combination. Our doctor preferred that ds be on formula so we could bump up calories, plus I hadn't planned to breastfeed. There were some mom's who supplemented with formula along with breastmilk. Sometimes CFers need more calories. We were given a recipe from the dietician for 27 calorie formula -- normal formula is 20 or 22 cal. Basically less water, more powder. One of the dieticians kept pushing a predigested formula for CFers call pregestimil -- very expensive and DS HATED it. I finally tasted it after he'd scream and fight every time we'd try to feed it to him -- tasted like a combination of how road kill smells and vomit. We immediately switched to regular formula -- think we used Infamil Lipil. Sometimes if I didn't want to mess with enzymes, I'd give him babyfood pears or bananas for a snack. Calories are calories.
Another thing to consider when the child starts solids -- babyfood is very low in fat and salt -- so we'd add butter to his babyfood oatmeal. DS hated rice cereal, but liked the jarred oatmeal and fruit. At daycare he ate regular food ground up. At home we'd add salt to his veggies and meat. And after he was 6 months old we added an ounce of cream to his formula. I also found some yogurt called yo-baby that was whole milk, but when I couldn't find that, I'd add a dollop of cream. Now that DS is a toddler we add a dollop of olive oil to his entrees, lots of butter, he drinks whole milk with carnation instant breakfast. And a hamburger happy meal at least once a week.