If you enable iCloud Photo Library, all of your photos will be uploaded to the cloud. Once you enable it, you have two options.
You can select "Download and keep originals", in which case a copy will be kept on your phone as well as in the cloud. This means that you won't have to worry about using any data to view your photos, but you will need to make sure that your phone has enough space to hold your photos.
Or you can select "Optimize device space" If you select this option, once your photo is saved to the cloud it will only retain a low resolution copy on your phone. That way if you go to your photos tab you can still see all of your pictures and scroll through them without using any data. It also allows you to have many more photos available than you actually have space for on your phone. However, with this option if you want to view a picture or share a picture, then it will need to retrieve the full resolution version from the cloud. By default this will use cellular data, however, you can also disable cellular data for photos and then it will only do that when you're on wifi. Obviously the downside there is that you can only see the full version of your pictures when you're on wifi, but you also don't need to worry about extra data usage.
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If you have seperate iCloud accounts then you will have seperate iCloud photo libraries as well. You can set up a shared photostream and manually select photos to put in there, but there is no way to merge your iCloud photo libraries (short of using one appleID for iCloud, but that gets ugly in a lot of ways)