If you check the admin cp demo here then you'll see under the 'Usergroup Permissions' link that you can change the permissions and allow/disable features on a per-usergroup basis, so that shouldn't be a problem to enable it only for your subscribers.
Upgrades for the gallery are about the same as upgrades for
vB. You upload the new files, re-apply any file changes (if you have made any, though none are necessary), and then run the upgrade script.