The automatic settings will randomly assign each student with a submission the specified number of peer assignments to review. However, once all the submissions are in, you can also manually assign peer reviews. In the right side menu of the assignment page there is a link to peer reviews. On that page there is a list of every student in the class, and you can use the controls below an individuals name to add submissions to review, or the right side menu to add submissions to everyone.
In your case, your post-grads need to be enrolled in canvas as students. If everyone is assigned to do a peer review, plus the post-grads, you will have some submissions reviewed more than once. The system allows that, however I am not certain how it will show up in student view.
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