How to stop multiple iDevices from sharing iCloud-sync'ed information?
I believe the best way to accommodate lush you goal is by doing the following:


Delete account from Settings > iCloud 
When asked to keep on iPad or delete, choose whether you would like that same information saved but be able to edit it without it affecting other device's information.
Choose set up a free Apple ID and proceed through the setup


The pros of this scenario:


Having the other devices using the new Apple ID to receive their own free 5 GB of iCloud storage 
You will still be able to use the other Apple ID for iTunes and App Stores
Edit the content from the old iCloud account without those iCloud devices being affected from the changes
Utilize the iPhone cellular calls with iPad and other features of iCloud which are functionally made to be segregated between people


The cons:


Another email / password combination to remember.


Other information: 


You can still share photos between accounts using iCloud photo sharing
Check the Apple ID used in Settings > Messages and Settings > FaceTime to ensure the proper Apple ID is being used, otherwise all devices could be sharing text messages and phone calls.
Additionally, you can setup Family Sharing in order to keep using apps, music and other media already payed for, and paying for future purchases with the same credit card.