Hinkjc asked what the chicks were vaccinated for and that's a good question that hasn't been answered yet. In fact, it's a key question. I'm guessing they came from a hatchery or feedstore which vaccinated them for Mareks. That being the case, the vaccination is irrelevant as medicated chick starter contains Amprolium which is a coccidiostat and is used only to keep the cocci already present in the chicks guts from multiplying to a level that is above their natural resistance.
Purina makes both a medicated chick starter and a non-medicated chick starter. I would suggest spending a little time on their website and reading as much as you can about their various products. I'm guessing "br" stands for Barred Rocks. So my suggestion would be to feed them either the medicated (providing they haven't been vaccinated for coccidiosis already) or not non-medicated chick starter. Purina's Chick Starter is specially formulated to not only have the right amounts of protein, crude fats, fiber, etc. but also certain amino acids and such for the age of the chicken.
That's not to say that you can't use Flock Raiser but I think you would see better results with the Chick Starter. Also, even though your chicks are only 7 days old, since you are letting them out to free-range during the day, they most likely aren't picking up a lot of cocci in their food or litter in the brooder. So non-medicated chick starter would probably be fine although the medicated chick starter was actually a buck cheaper than the non-medicated the last time I bought it.
Hope that helps.
God Bless,