I know this is pretty simplistic, but read everything at www.morethantwo.com
before you get started, especially the stuff on poly configurations and jealousy management, as well as the basic tips and do's/don't's, and you will be better prepared for surprises that still come your way (and there will be surprises!). If you're committed to a primary/secondary structure, don't look for exclusivity from a new female partner (I wouldn't suggest doing so even if you weren't, but especially in this case).
It sounds like you guys have a strong base to work from and an understanding that flexibility is an asset when moving into new territory, so you're well equipped already. I would just suggest arming yourself with more info and perspectives, as per above.
The major players. Me, 30ish bi female. Gia, girlfriend of 4+ years. Clay, boyfriend/dom. Davis, ex/friend/"it's complicated." Eddie, roommate & fwb.
The supporting cast. Eric, Gia's husband. Bee, Gia and Eric's toddler. Dexter, Gia's lover. Helen, Eric's lover. Izzy and Nikki, Clay's partners. Liam, Eddie's husband.