How to delete photos only from one device? As I have limited storage on my iPhone, can I delete photos from my iPhone and they will still remain on the iCloud? Or can they be accessed on my iPad which has more storage?

I think when I have deleted a picture on any device, they delete from the iPad as well (presumably through the iCloud).
As detailed in Apple's support article, you can disable iCloud Photo Library on your iOS device by tapping Settings > iCloud, and turning off "Photos".

Your photos will still remain in iCloud Photo Library but will be removed from your device.