Hey 1st! Yes, in almost every case your spouse will be your primary. I've always thought this summed it up best (I'm paraphrasing) -- if your primary gets an amazing new job across the country you start looking at home prices, if it's your seconday you start looking at plane trip prices, and if it's your tertiary you say "Call me as soon as you get back into town to visit!" They're descriptors for the roles people play in your life and the degree of entanglement and prominence. I've probably erred in my blog once or twice in calling Davis primary to me... it would be simpler and more likely for me to get there with him than with Gia, but right now he and I are really still casual enough that it's not quite appropriate. Anyway, I hope that helps clear things up.
I think you're doing the right thing by doing a lot of reading here. Also, check out this site if you haven't already it's awesome: http://www.xeromag.com/fvpoly.html
Poly situations can create so many complications, misunderstandings, hurt feelings, etc. Be careful, thoughtful, honest, and considerate, though, and you should be ok!