Hi Brad,
There are a couple of ways to do this, depending on your needs:
You can utilize the container document's introduction to notify users which prerequisite guides are optional and which are mandatory.
You can adjust the testing routine guide itself to say "optional" however you like.
If you need to use the same testing routine guide as a mandatory prerequisite sometimes, and an optional prerequisite at other times, you can clone the guide and make one mandatory and one optional guide to use as you like.
Please feel free to follow up, you obviously have specific applications in mind for Dozuki and I want to facilitate them.
Best Regards,
Chris Burkert