Just my two cents worth - as you are young enough to be my daughter, I would encourage you to wait a long time before doing the walk down the aisle thing (as you mention fiance). With the housekeeping out of the way - I am so glad for you that at 20 you have discovered there are alternatives to being mono, and have comfortably identified yourself as being poly.
Don't worry about whether your partners are mono or poly, it is something they will need to figure out for themselves.
You have an advantage I did not - the ability to go into a primary relationship with the poly cards face up on the table. In my case, it morphed into it and my bride is not overly pleased.
I would suggest some reading aloud with your partners. Our favorite is "Opening Up" and be taking turns reading and then discussing, it is both very intimate and lets each person know they can openly and honestly discuss any topic.
I'm also a firm believer in my partners knowing each other, and if not being friends, at least get along with each other.
I'd agree - my having an OSO has caused me to love my wife more than I ever felt was possible.
Ciao ~ Mike
Blessed are the flexible, for they shall never break.