Noob question: I want to use the iCloud family plan for my family of 5. How do we prevent getting each other’s apps, contacts, etc when we backup our individual devices? Is there a way to separate our individual info from each other on this plan?

How do we prevent getting each other’s apps, contacts, etc when we backup our individual devices?

As long as you each use a unique Apple ID, your content, including backups, will not be shared. You are only sharing storage, not content. There are a few shared things, like a calendar and a photo album and such. It's not everything.

Backups remain separate for each device and include only what’s on each device, not everything that could possibly be shared. Each person’s iCloud (assuming each has a different Apple ID) also remains separate.

What's in iDevice backups (contents)

https://support.apple.com/en-us/HT204136