Can you explain more about "free some space"?
Since you have "Optimize Photo Storage" enabled on your iPhone, your iPhone will only store low-resolution thumbnails of your photos on your phone. These don't usually take up much space. When you tap on one of the thumbnails, your phone will download the full version of the photo, but it only stays around temporarily. You don't have to worry deleting those "to free up some space", your phone will do that automatically for you.
If you ever need to zip up photos (like to email to someone), you can use
www.icloud.com from a computer. You'll see all of your pictures, ... you'll be able to select the ones you want, and when you download them, they'll come down in a zip.
If you have iCloud Photo Library turned on, then the master copy of all of your photos "lives" up in iCloud. Any change you make to a photo (like an edit or delete, or organizing them in albums) on a device (like your iPhone) will sync up to iCloud, and then sync back down to your other devices (like your computer, iPad, etc).
It does
not allow you to pick and choose which specific photos to sync down to a certain device.
My iPhone is 64GB, and most of it is used for apps and music. My Photo Library is 104GB (40,000 photos and videos). I can access every photo and video from my iPhone. My phone's never run out of space (even when I take a bunch of new photos and videos). For me, iCloud Photo Library has been doing a great job of managing space on the device (by itself).