You can also remove photos from a stack by tapping on the “Remove from Stack” option. If you want to keep one photo from the stack and delete the rest, you can tap on the “Keep This, Delete Rest” option.
Google has not yet officially announced a release date for Photo Stack, or if that will be the final name for the feature, but it will hopefully roll out to all users in the coming weeks judging by the advanced state of the functionality at this point. It also remains to be seen how helpful this feature will turn out to be, as it will introduce yet another way of organizing photos without users having to manually create albums.