How have you been using iCloud on your older devices? When the iPod 4th Generation has been introduced in 2011, iCloud has only offered My Photo stream and one year later Shared Photo Streams. That has not been a permanent storage for photos. You might be finding some of the very early photos the Shared Albums, the new name for the old Shared Photo Streams. Anything you have saved using Mobile Me, the predecessor to iCloud, is no longer available. Apple cancelled Mobile Me in 2012. And My Photo Stream did only store the photos for at most a month. It is no longer available either: An Ubituary of My Photo Stream - Apple Community
If you have been syncing your devices with iCloud Photos, when it has been released in late 2014 and kept paying for your iCloud subscription, you may want to check your iCloud Photos Library on your iCloud website on a computer:
Open www.icloud.com in a web browser on a computer, then sign in with your iCloud Apple ID, open the Photos.app on that web page. All photos that are still in your iCloud Photos Library should appear on that web page.
iCloud Photos is only storing the photos you are syncing from one of your Apple Devices or uploading using the web interface. if you delete photos or videos from one of your synced devices while they are still signed into iCloud Photos, the photos will be removed from from iCloud and all other synced devices as well.