Yes, I missed that.
You can share to devices not running High Sierra, but to initiate sharing it must be done from devices running iOS 11 or High Sierra.
To get more storage just buy more the same way you did originally. Just select a larger plan.
But again, the Photo Libraries would only be different if you and your wife are using different Apple IDs for iCloud.
If you are using the same one, you aren't sharing the storage as much as sharing the account.
If you turn on iCloud Photo Library on a second device with the same Apple ID all you are doing is merging libraries.
Having different Apple IDs means each person gets a different iCloud Photo Library with their own photos.