We got a new iMac and would like to enable iPhone Photo Library on it so it can download photos onto it from our existing iCloud Photo Library. However, since this is a new computer, our Photo library on it is empty, so before I enable iCloud Photo Library, I wanted to make sure my iCloud Photos are safe.
If we enable iCloud Photo Library on our new iMac with the empty photo library, will it download our existing photos or will it try to sync the empty library to iCloud and possibly wipe out our photos?
You can safely enable iCloud Photo Library. When you do that, Photos will always treat the iCloud content as canonical. It will download everything from it to the device, and if that device has additional content, it will upload that content, merging it with iCloud, so that new content will also be available to other devices as well.