You are correct. I am scheduled for a Customer First, non-reveal shop, and the instructions include this stipulation:
"Also, if you are already qualified as a revealed auditor for Chevron Image, you are ineligible to accept
mystery shops for Customer First."
However, despite that blanket sounding statement, I don't see why you couldn't do a Customer First shop if you have never visited that particular location even if you do reveal shops elsewhere.