My school has an eight period day (period 5 is lunch & a class). Anyway, hypothetically speaking, if I were to take only 7 credits (that means 1 study hall), would colleges view that unfavorably compared to someone who took 8 or even 7.5 credits (assuming we both get similar grades)?
No. It’s fine to have a study hall. As you spend more time on this site, you will see that many senior posters don’t advocate being an academic drone. Trying to cram academics into every period isn’t going to be a boost, unless it’s something like science or humanities research, which might mean you have to give up a study hall. Colleges want to see course rigor, and its fine to have study hall so you can get homework done, or study, to ensure good grades.
It can depend on what class you give up for a study hall or empty period, but with 8 class periods, scheduling classes in 7 of them should give you plenty of opportunity to take the expected “core” academic courses plus some electives.