I don't know what Greenfields policy is, but my agency forbids making traffic stops with non-personnel in the squad (with the exception of sanctioned ride alongs).
Most agencies allow plain clothes officers to make traffic stops, but it isn't something that should be a habit. One is within their rights to request a uniformed officer in a marked squad to respond. But unless you're sure that the person is not a police officer and is impersonating do not just drive off. Doing so shifts the burden of justifying actions to you.