Your Fisheye and Crucible (Fe/Cru) user tiers do not need to match your JIRA licensing tier in order to integrate the products. Only users who are given permission, through either the JIRA user management or internal Fe/Cru user management, will count against your Fe/Cru licenses. 

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 Fe/Cru users and give only those users the right to log into Fe/Cru. I suggest reviewing Connecting to JIRA for user management for more information.