You are sure going to have your hands full! I wish more moms of multiple kids would try breast feeding but so many give up early due to how many kids they have on hand. You may find with 5 kids pumping logistically difficult. If you end up having to use formula, it will be ok. With so many kids at home, the younger babies do get exposed to more crud which often overcomes the benefits of breast feeding - like having your own daycare at home.
In my practice, which is a huge multi-center practice in Massachusetts, we've taken a more moderate "a growing baby is a healthy baby" approach so moms who can't or don't want to nurse don't have any guilt. In the end, a baby who is growing well and loved will thrive, even if not given breast milk.