If you are using iCloud Photo Library on all the devices, then the library is synced, and photos deleted on one device will be deleted on all. If you are not using iCloud Photo Library, then what happens on one device should not affect the others.
If you turn off iCloud Photo Library on the iPhone, it should give you a few options. I’m not sure what they are, but as long as you don’t choose something that says permanently delete photos from all devices or the library, you’ll be good. However, if you re-add the phone to iCloud Library all photos will be resynced.
If you turn off iCloud Photo Library on you iPhone, I recommend turing on “Upload to My Photo Stream” on both the iPad and iPhone. This will let you view the 1000 most recent photos taken on both devices. Keep in mind that photos that you take on the Phone like this will not be permanentlhy saved off the iPhone unless you select and save them to the main library on the iPad.
Optionally, you can turn on Optimize iPhone Storage in Settings > Photos.
I have about under 7600 photos in my iCloud Photo Library. In my iCloud Storage this take a little over 28 GB. On my iPhone, with the Optimize setting on, it only uses 1.5 GB of local storage. This can vary a bit, as recently viewed photos will be maintained at full size for some time before being optimized again. It also means that if you have no internet, you will only be able to view somewhat degraded versions of the photos.
Also, if you have Photos enabled under Settings > Wi-Fi, use of the Photos app when away from wi-fi will increase you’re cellular plan usage. Opening older photos for full screen viewing will automatically download the full version. You can see this in action, as the photo will suddenly improve shortly after opening. There is also a small download progress icon in the lower right (or maybe left) of the screen)
