Does my Stash licensing tier need to match my JIRA user tier?
Your Stash user tier does not need to match your JIRA licensing tier in order to integrate the two products. Only users who are given the 'Stash User' permission, through either the JIRA user management or internal Stash user management, will count against your Stash license.
If you plan to use JIRA for user management, then you'll have the ability to create a group within JIRA to contain all of your Stash users and give only those users the right to log into Stash. I suggest reviewing Connecting Stash to JIRA for user management for more information.