"I also wanted to ask what kind of relationship you all have with your partner's significant others?"
Poly has a wide range of shapes and sizes. My arrangement is kind of a closed poly-fi V. There are two men and one woman, we're all hetero so the men are just friends. Not really looking for anyone new to add to our circle, although it's theoretically possible.
It takes awhile to recover from a road bump, and you're right, clear communication is paramount.
Way to go on sending those emails. I'm sure it will help ease some of the fear of the unknown.