Each AppleID account has a basic 5GB storage quota. Having multiple devices does not increase the free 5GB storage quota; one device or ten, you only get 5GB.
While using multiple AppleID accounts is certainly possible, in most situations this is not recommended. Using a different AppleID accounts can introduce many complications and issues, not limited to losing access to paid Apps, subscriptions and services - these being forever tied to the AppleID account from which they were purchased.
If you do wish to change and use an entirely different AppleID with a device, first ensure that you have disabled the Activation Lock that will otherwise cause difficulties in the future. You can disable the Activation Lock by disabling the Find My iPad service from iPad settings before trying to use a different AppleID account:
Settings > [Your Name / AppleID] > Find My > Find My iPad - set to OFF
After disabling the Activation Lock, you can either sign-out from the iPad - or simply reset the device. Neither should impact data stored on your iPhone.