Although after deleting the images from the Cloud, you can sync them again from Classic which will upload Smart Previews to the cloud. Those Smart Previews do not count against your cloud space allocation, so you'll have them back on the iPad in due course without any space penalty. Smart Previews only of course, but usually plenty good enough for viewing and even some editing....just remember that you can't export the full-size original from any of the "cloud" apps.
N.B. it's safer to do the initial deletion from the Cloud by removing them from the All Synced Photographs collection in Classic, which retains the images in Classic but removes them from the cloud. Doing it like that ensures you can't accidentally remove images from the Cloud before they've downloaded into Classic.