You are correct, Rosemary, that Apple does not permit App Store apps to read more than the system photo library (because the only way to do it is via private API calls which are forbidden (and a bit risky as well, since Apple makes no promises about them).
You can switch the system photo library without messing up iCloud. Here are two links:
Multiple Libraries: https://www.youtube.com/watch?v=nR0wRXPg7Tw
Apple Support Page on multiple libraries: https://support.apple.com/en-us/104946