The OP already has a gf and a bf and they are designing the commitment structure around already existing partners.
I would imagine just like in any other relationship, you wouldn't have "the talk" until you've been dating a while and feel ready for more intensity or exclusivity. By then you'd have a good sense of where the person you've been seeing is at. I guess if I ever got to that point with a woman, I'd ask her "Are you interested in seeing other women, or can I expect that I'm your only girlfriend?" or something along those lines. Of course, that would be unrealistic if I was dating, say, one half of a lesbian couple, or a woman not very interested in men and not content to be my "secondary." What draws me to polyamory is the idea that you can be committed, in whatever way makes sense to your situation, to more than one person.