As you're pondering your science options, I will share that a more typical sequence would be to do as follows:
9th grade Either Physical Science or Integrated Physics or Chemistry (IPC is preferred by most states over physical science)
10th grade Biology
11th grade Chemistry
12th grade Physics or another advanced science
You could still follow this sequence by simply borrowing the science from the guide ahead of the one you are doing each year.
This is what I've been thinking - doing science from the guide ahead. However, we're on a very limited budget and have four students to plan for. There is no way I can purchase both the MTMM guide and the Geography guide. If the science was a separate guide, then it could perhaps be purchased separately at a more affordable price. I know you've mentioned before that it wasn't in the plans to do it that way; however, given this suggestion, perhaps it could be revisited?