Hey there! Its good you guys are thinking of the future now. I would suggest striving to stay as flexible as possible, as what you find/feel as you go along may well surprise you. More talking is more better, so kudos there! Hopefully the gf will come around to opening up to you.
One question immediately springs to mind when you mention a polyfi triad/quad. Does that mean his new gf is to have no other partners? If so, that seems a little unbalanced, as he has her and a wife.
Some other thoughts... what would bringing her into your family mean? What if she wants to be in a relationship with him but not "join" you guys, whether that means adhering to the relationship structure you have in mind, moving in, or otherwise becoming more integrated into your lives... would that be ok?
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.