There are many threads on here about mono/poly dynamics and i suggest that you do a tag search to find them.
I don't advocate for mono/poly relationships, truth be known. I have not found one example where all concerned are perfectly happy and understand one another. There are huge compromises and I have not seen any way through them to become solid boundaries in my own personal struggle. I do know relationships that are working for now, but the future is always unpredictable in mono/poly... with poly relationships love is added and not generally taken away. Relationships change and morph into something else, but generally no one leaves unless there is some huge reason. In mono relationships, when it is over, its over. Things are far more black and white. Trying to understand both sides of that is next to impossible.
That being said, if I were you I would not make any promises you can't keep. Be perfectly honest and realistic; what hurts now will hurt a hell of a lot more later. Know that there is a possibility that you are going to hurt your partner like no one else has. They will also cause you pain too as you make compromises to be with them.
I would suggest going very slowly and exploring if there is a chance they are poly also... at least in theory. They don't have to practice it to be it. There might be a chance that an understanding can be developed out of that.
What has worked for me is to prioritize who I want to be with and then work towards boundaries that reflect that. I hate having to do that, but it has been necessary. At this point in my poly journey of 15 years I am at a place where I see no boundaries between people, no hierarchy of primary/secondary etc. I just see and feel love for those I resonate with. I want the opportunity to let that go. But mono/poly relationships don't allow for that. At least if there is any depth and connection... if it's just dating then perhaps, but it sounds like you are established and have connection.
I wish you well, perhaps if poly is all new to you then you will have a chance to create something from scratch that works for you both. Good luck...
(sorry this is rather defeatist. I assure you its my own shit that is guiding me right now. I hope that others that are a bit more positive will chime in... still, I figure that knowing everything honestly is better than a glossed over version that makes it sound like it will all be chocolate and roses... hope you get that)
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