I agree that it's unrealistic to expect your wife to not only agree to you seeing someone else after years of monogamy, but also drop all her expectations of primariness right away. For most people, being "primary" in a partner's life is the definition of "being in a relationship." It's a big change to be "one of a number of important people."
I think hierarchy doesn't work in the long term -- your new partner will eventually feel restricted and demeaned by rules that say your wife always comes first or is entitled to certain things because of her wife status. But using your philosophical objection to hierarchy as a justification to let your NRE go wild will be hurtful to your wife, and, ultimately, to you. Non-hierarchical does not mean "ignore the needs of the existing partner" -- if you love that partner, she should naturally come first for quite a while, until your new relationship becomes more established and earns an important role in your life.
Even in monogamous dating, I've always tried not to spend too much time with a new partner, because I knew that I couldn't really know him yet. Getting our lives really intertwined and putting less time into my friends and hobbies to spend more time with someone who might turn out to be temporary didn't make sense to me.
In your case, you've got a wonderful relationship with a wonderful person. If you take things slow with your new person, your wife may be able to adjust and to feel confident that you'll continue to make her and your child a priority. Perhaps she'll find that she appreciates having some time and space, or the new perspective that you bring to your relationship, and she can be happy with your poly situation. But if you rush, your wife won't have time to adjust. She'll just be trying to tolerate your other relationship, and your relationship with your wife will almost certainly suffer. Sacrificing a marriage just because you're feeling NRE is something you'd likely regret.