Once you delete photos off your device, will they also be deleted from iCloud?

If using iCloud Photo Library, yes. iCloud Photo Library is a synchronization service. It is not archival backup storage. Any changes made on any connected device will be reflected in iCloud Photo Library and across all other connected devices.

Before deleting anything, transfer those photos to archival storage - your desktop or laptop, or an online storage like Dropbox or Google.

Hi,

No, but you must be sure that your turned off sync photo in iCloud account in your phone.