Hi, thanks for posting!
Yes - The ExclusivelyUsesAttribute should work at fixture level, and it should also inherit through to derived tests.
There is a known problem with NCrunch where sometimes if you add a new ExclusivelyUsesAttribute onto a base class that is in a different assembly from the test itself, you'll need to reset the engine before NCrunch will properly pick it up (or just restart Visual Studio).
The best way to tell whether the attribute is working correctly is to turn on the 'Exclusively used resources' column inside the Tests Window. To do this, just right click the column headings, go to the 'Field Chooser' and drag the field onto the tree list. In theory you should see the resources being used by all the tests in each of your fixtures, but note that the fixtures themselves won't actually make use of the attribute (they implicitly inherit the attribute from their child tests, and this won't be shown in the tree list).
Thinking about the error message you're getting - make sure that the database you're testing against is properly isolated for the continuous test runner. If any other test runner is working from this database, or if you are using it for manual testing, you'll likely experience problems as described.
I noticed that in the example above, the 'ConcreteIntegrationTests' class wasn't inheriting from your base class .. but I assume this is unintentional and that your code is right.
I hope this helps!
Cheers,
Remco