Q: iCloud storage

Hello

Can anyone explain how to manage iCloud Storage ?

I have several devices

MacPro Desktop with 9 hard drives for video and 10 photo libraries

MacBook Pro

iPhone

iPad

All I want is the iCloud to be able to

A) back up documents

B) take pictures I shoot with phone or iPad and safely back them up

But when I back up the phone or iPad, iTunes or Photos tells me it is going to delete all the current photos off the phone to sync with whatever library I've got running

It keeps telling me I am out of space in the cloud but I want to pick what I need backed up and that is much smaller than the 5 Gb Apple provides

Q?- How can I create a folder that has a fixed set of pictures on my phone and then be able to add, download and erase new ones without deleting the fixed set.

many thanks

Enio Cordoba