Well, I can't promise you it's going to be fine -- no emotional relationship comes with that guarantee -- but it definitely does sound like you're overreacting inasmuch as you found exactly what you want and are freaking yourself out over unknowable stuff. Give it a go, this guy sounds great.
There are resources out there to help if you have questions or get jealous. Let me suggest the essays about being a secondary partner (limited time and limited blending of lives) at www.morethantwo.com
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.