Do you need to integrate Crucible with CVS? If so, then you'll need FishEye or develop your own custom Crucible SCM plugin. If you don't need CVS support, then you'll be able to trial Crucible on it's own. Did you configure your SCM integration via FishEye or a Crucible SCM plugin? If you configured just the standalone SCM plugins, then you're already trialling Crucible without FishEye.
If you configured repository integration via FishEye, then you can trial Crucible on it's own by removing all repositories from FishEye and adding them back via Crucible SCM plugins. This will disable FishEye.