Is it safe to breastfeed one of your children, but give another formula?

Breast vs bottle: It is safe, and both give the baby the nutrition that they need. However, breast milk is uniquely designed for human babies. Most formulas are cow/soy based. There are other things in breast milk, not in formula, as well. If there are circumstances that make you need to use formula; it's fine. If you have a choice, I will always recommend breast milk.