Where a formal hearing is not required by law and where the parties agree in advance to proceed in such manner, the agency shall acquire the information upon which it bases its decision by means of informal conference or consultation among the parties as follows:
(1) The agency shall conduct the conference itself or may designate a subordinate to do so;
(2) The parties may appear in person and by counsel; and
(3) The parties may submit any relevant factual data, documents, testimony and argument. Only such evidence and argument presented at such conference or presented to the agency and opposing parties before the conference may be taken into consideration by the agency in making its findings and rendering its decision.