Why does anyone have to be a primary? Can't several women or a couple of women be a priority? I think you have this a bit muddled. You are talking about another person here. Another woman that you say you will likely love along with have sex with. Why should she get the bits left behind from a primary relationship? Not very fair I don't think. I think what is important to realize is that this fantastical woman has just as much rights as your girlfriend does. That doesn't mean that she will fit the same spot as her, it just means that she is entitled to have her own boundaries respected, her own agreements with you, and just as much respect.
I think it very unwise to think of this from the perspective of swinging. Your statement about "looking for a woman to experiment with my girlfriend and me" is a red flag to me. Who says that your gf would find said woman attractive? Who says that the woman would find her attractive? Any number of things could come up when emotions occur. Sex? Whatever, its doable under the concept of swinging. Just don't fall in love.
This route you are suggesting is not the easiest route and not a good way to start seeing if poly works for you I don't think. Neither is swinging if you are prone to emotional connection with those you have sex with. I suggest you scroll right back and lay it on the line for yourself and her. If you want to be free to love others and have sex with them then tell her that and stick with it. It sounds like she is not interested and there is no reason she HAS to be involved with anything you do in terms for finding more love in life. Her role is to think that is awesome and support you while you support her in what she does. That to me is how any relationship in poly(or mono) works best. I would start practicing this.
If you read around here you will find years of people posting with similar concerns and thoughts. Have a look around. Read other threads. Do a tag search for "triads" "unicorns" "lessons" "foundations" "secondaries" "boundaries" "swinging" and anything else that seems interesting. I think the biggest thing missing from your story is education... yours and hers.
Take your time. There is lots of time. Keep going to that group. Age means nothing. Think of it as getting to know like minded people not as a place to find dates. Lots of older poly people could teach you some shit! Besides, you might find someone hot if you get to know them. I think that poly party was a great place to start.