We are currently using Jira (self hosted, not on-demand), and are about to purchase FishEye and Crucible licenses.
Since only developers will be using FishEye and Crucible, we don't need as many users for it as we need for Jira.
I wanted to make sure we can purchase FishEye and Crucible licenses with a smaller number of users from the Jira licanse, since I know some add-ons require to purchase the same amount of users.
FishEye and Crucible licenses can be bought completely separately from each other and in fact from JIRA licenses.
ie. you can have an unlimited JIRA license instance connected to a 25 FishEye license instance, and even a 100 Crucible license instance if you wish.
There is a neat option in FishEye, when you configure the authentication mechanism (can be to your JIRA instance if you wish) to allow a user to be added to FishEye only when that user logs in to FishEye. This means, you can connect FishEye to a JIRA instance that has 1000 users, and only have the 25 users who log in to FishEye consume your user-count.