I’m glad you recognize that there could be a conflict of interest here, especially if the candidate is unqualified or borderline.
You don’t need to hire reviewers, but if you’re going to use candidates to review proctors, I’d make sure that at least two candidates in the same administration were surveyed on the proctors’ performance. Have them get a review sheet prior to the administration, not take notes during the administration, fill out the review sheet after the administration, and submit their reviews independently without interacting. In fact, they should not even know there is another reviewer in the test administration area.