Would you have to amend the permissions page for who can 'use' and 'administer' so licenses are not automatically used? For example, if jira-users have 'use' permissions. Do I need to add confluence-users group to the revleant users in Jira and remove jira-users permission?
That depends, you can use your own user management in confluence or delegate it to jira.
But in general confluence-users and jira-users are two different groups and there should be in your case no relationship between these two groups. Just add the 15 accounts to confluence-users.
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