Is it possible to delete selected photos and videos from pixel 2 which have been backed up .... If possible then how can I watch those images and videos if I delete them from phone ?
If you backed them up to Google Photos then yes you can delete them (can wait for the prompt in Photos to ask you so you know the backup was completed), to view them again you would look at them through the Photos app to pull them from the cloud. Any videos would be streamed from there as well, so will use mobile data if you're not on WiFi.
Open Google photos app -> swipe from left edge to bring up options. There you can click "Free up space" option. This will remove device copy for the photos you have backed up. Those photos still show up in the photos app normally but they will be in the cloud instead of the phone. You can also download any of the photos to the device from the same photos app if you want them later to access locally.