I think that is just fine. As long as you are able to get all of your thoughts and feelings out, you just might feel better after. I know you have had a lot of thoughts weighing heavily on your mind. You may have already addressed it, but have you asked him why he is so confused on what he wants with Molly? I know within minutes of meeting a person what role I see for them in my life. You mentioned that you are not quite sure he is poly, and she is mono and not quite a fan of him being poly.
If you are intimately and seriously involved with someone who does not believe in poly, and who will resent you for also being with someone else, this will create an unhealthy environment not just for you but for me as well, because your confusion about your relationship with her spills over into our relationship.
I would put emphasis on this because it is true, and it needs to be stressed. If Molly is having issues with him being poly or possibly, you are going to feel it in your relationship. It may come in the form of losing balance and him spending less time with you. It may come in any form, but as you are seeing now, it is spilling into your relationship. He needs to know how his confusion over things with Molly is creating problems in your relationship with him. He might not even be aware and thinking, "Oh. This issue is just between Molly and myself. There is no way it is hindering my relationship with Cleo." That kind of thinking is wrong.
Other than that, I hope the talk goes well. Be firm and open. Good luck.