Can I download a certain date range of photos from iCloud on to my iPad?

No. iCloud Photos syncs everything to iCloud, and then syncs everything back to any device that is logged in with the same Apple ID.

There is no way to select specific photos to sync down.

The Optimize Storage option in Settings ➜ Photos will reduce the quality and file size of photos and videos stored locally if the iPad is running low on space, but its done automatically and there's no way to specify photos or date ranges for it. Though it should keep more recent photos and videos in full quality and file size over older ones you have not accessed in a while.

click here ➜ Use iCloud Photos on iPad - Apple Support

You can always transfer photos and videos out of the iPad and onto an external drive or computer and delete them from the iPad.

click here ➜ Connect external storage devices to iPad - Apple Support

or here ➜ Transfer photos and videos from your iPhone or iPad to your Mac or PC - Apple Support