"Heres the thing, I can't risk a dog go rogue and bite or terrorize someone - and I realize that I didn't pick the best candidate to start with."
To be honest, almost no dog goes rogue, there are always signs- just we miss them sometimes. And trust me, no one likes the idea of their dog biting someone! But its a risk with ANY dog, just some have a much higher threshold and for different things/situations. My dog for example EXTREMELY low threshold for strangers, due to fear.
I agree with what the other to are saying, with too much too soon.
Also, remember this. Dogs don't talk. They communicate via body language, growling, barking, howling, etc. Growling is just another communication tool they have to use with each other and us.
Honestly, he sounds like he had an insecure moment (new people, new home, he just came from a stressful shelter, who knows his history, he probably has a lot of pent up energy and with no mental stimulation, etc.) and with "too much too soon" its not really a big red flag to me that he growled. Just that there is a lot to do to set him up for success, but it's not HARD stuff to do!
I would also advice you to get a trainer with good reviews, who can help you set the dog up for success in your family. Giving you both structure, daily routine, and for you to learn how to read your dog.
Also, I do not know him personally so this is just me basing off what you had said about him and the situation. He sounds like he is going to be a great dog though with structure and training, and to make sure he feels secure- that you "have his back" and he doesn't have to worry about anything.