If Idelete my photos from my device, will they still be in the iCloud to download to another device?

If you are using iCloud Photos and you delete a photo, it is deleted everywhere. It will remain in the recently deleted album for 30 days and then it will be gone for good. iCloud is merely a syncing service, not off device storage.

If you backup photos with iCloud and you delete a photo from your phone, when the phone backs up again, that photo will also be deleted because it is no longer on the phone to be included in the backup.