Is it true that if I let Google Photos store all photos, then I can back up to iCloud with minimal storage?
There are 4 questions here.

How can one answer 1st one without going to opinions? If you don't feel that those notes etc., are important, better to save bandwidth and space. 

2nd also depends on how much is your rest of the backup. One can also have no documents, huge media collection or vice-versa. I am running 5 GB and I put less urgent/ important photos on Google Photos. iCloud is used for documents also but they're more or less distributed over accounts. 

3rd: It's up to you to decide. You can test and verify which settings work best for you. If something piles up, decide whether you really want it synced or not.

4th: I doubt that all emails(in non-Apple accounts) are backed up by iCloud. Only iCloud emails count against iCloud Drive quota.