How can I limit a workflow rule to tasks with FTS predecessors?

What did you try that did not work? Were you able to find the field that specifies the dependency type?

No, I couldn't find the field to add on the conditional action test.

You would probably need a custom checkbox field at the task level and then a workflow rule running on dependencies that would check this field on a successor if a FTS dependency gets created (and a secondary rule to uncheck it if the dependency gets deleted). The validation rule would then not allow tasks from concluding if this is checked. Below is an example of how you would check for FTS dependencies when running a workflow rule on the Dependency object.

As a side note, there is also a concept of "executable" tasks, which means tasks with no active predecessors. You can also create a validation rule that prevents tasks from being completed if they are not executable.