Open minds, open hearts, open home.
It sounds like both you and your husband's hearts are in the right place. I can understand everyone expressing concern over the idea of everyone moving in together right off the bat, but I have to say that I don't think that any of your story strikes me as a "predatory" concept.
I have helped friends in the past by having them come and stay with me while leaving relationships that were no longer serving them. Have helped friends in rough patches by having them live with me for a few months, or just enjoyed helping them save for their first house while co-housing with me. Even if I was having sex with any of them (which one of them I was) I don't consider that predatory - I consider that living together, and having a relationship at the same time.
I think it takes a lot of courage to consider polyamory when you've been in a monogamous relationship for a long time. Kudos to you for not being totally closed to the idea, and for seeing your husband's ability to love another human being while still loving you. Obviously there are lots of questions to answer, and things to figure out, but in the meantime, don't allow people to bully either of you away from your truth - there are people with very strong opinions on this board and it can catch a person off guard at times!
She *is* a fully grown woman, which means that she is capable of choosing who she wants to be in a relationship - and who knows, she may choose your husband and a shared life with both of you. While it makes sense to consider larger life questions (like having a child, or living together) as a general concept, my only advice would be to take things slowly, and concentrate on communication and exploring what is in the here and now. I can totally relate to getting too far ahead of myself in relationship, and once I take my foot off of the gas and look around, I always feel much more grounded.
Good luck, and keep posting!