Well, it's not worth the rift between you and your husband.
She's a beautiful girl, and I can understand why you're agonizing about this. I don't know how important it is that she's lived with other cats. We had six cats when we rescued our seventh. He was a very, very cat-friendly cat. And our other cats were a little older (he was almost two: our youngest at the time was 3, we had one that was 4, and the rest were 5). Billy came inside, happy as a lark, and tried to be friendly to all our kitties. Our gang was nice enough to each other, but no one slept cuddled up to another, they didn't groom each other, etc. So when Bill tried to be groomed, groom them, get close to them, they all reacted to him like he was some kind of alien.
Cats are all about territory. So when you bring a new cat into your home, especially when they're older and more set in their ways, the introductions are best done very slowly.
I don't know if your husband will change his mind about the adoption if he understands that if you do this slowly, sensitive to your kitties' needs, there most likely won't be a problem. But there is a right way to do it, and you take it at their pace.
...and IMO, "introductions" is a bit of a misnomer. When bringing a new kitty into one's home, the focus really (usually) isn't so much on - or shouldn't be so much on - the kitties "meeting" each other and "getting to know" each other. It's about them establishing a new kitty hierarchy, and learning how to share the space.
If you have a spare room with a door, she can most likely be adopted without a lot of stress. The room should be set up for her, and going into it assume she'll need to be separated for some time. A week? A few weeks? A month? Six weeks?
However long it takes them. And no need to feel guilty about her being confined to one room (so long as you're prepared and able to spend time in there with her - even sleep in there sometimes if you can
). It's much better than a shelter cage, and won't be long term.
But this gives your two the time they need to get used to the idea of another cat being in their territory. It gives everyone time to get used to each other scents (and you can help with that). It gives you the time to get them associating each other's scents with "good" things they love - which helps ease the transition of the new kitty into your home.
One thing you can do is take a clean rag - preferable it if hasn't been washed with scented soap or fabric softener or dryer sheets or anything. Take it with you to the shelter, and give her a real good pet with the rag, ensuring you rub her all over with it. Take it home, and let your kitties sniff it. See how they react to it. Some cats will hiss, some will just sniff and sniff and sniff, but express no negative or positive reaction. This doesn't mean everything will go swimmingly. But if you get a hiss out of it, it's probably not a good idea to adopt her, given your husband's fears.
As to introduction tips, I have quite a few, as do many on the site.
But it's probably not the time or place to bore you with those now.
Just FYI, we rescued our last kitty in the summer of 2010. He was an older feral cat, so aggressive outside all the colony cats disappeared - and didn't come back until he was no longer outside for three days.
That's why we took him out of the colony, he was just too disruptive. We live in an RV (yep, with seven cats at the time!), so we have no way to separate new introductions. He was so food aggressive, we figured he'd calm down with all-the-time access to food. He was in poor health, and pretty beat-up, so we also figured he wouldn't freak out being inside. Thankfully, we were right on all counts. We rented a trailer, put it next to us, and put him over there. He adjusted to inside-living while we worked on socializing him. He's a smart boy, and figured out pretty quickly we meant no harm to him.
Most of our cats were 8 years old then, and it was already pretty crowded in here, so we were kind of anxious as to how things would go. We did a LOT of scent swapping before we ever brought him over here. It was about a month, I think, before we brought him over here. We turned on the engine of the RV (which sends everyone fleeing to the bedroom) and closed off the back of the RV. We put Chumley in a crate and brought him over. We let him out to investigate for 15 minutes. That was it, Then we took him back to his trailer. Our cats came out, and could have cared less that he was here - but they were used to his scent in their space at this point (we'd brought over beds and blankets that smelled like him and replaced beds and blankets they used, and we'd brought over poop and pee of his and put it in their boxes, so they were used to his scent in their boxes). We brought him over the next day without closing anyone off, but at a time when they were all sacked out. He cautiously sniffed around when out of the crate. One by one the kitties noticed him but ignored him, which was exactly what we wanted.
We just continued to do that. Bring him over for a little longer, and a little longer each time. While he was here, we would play or pet or brush the other cats. We essentially ignored him. While he was here, the attention was always on the other cats. We basically ignored him - unless he joined in the play. Then he was here long enough for a meal, and we just included him - putting his food down last. It only took three weeks, and then he just never went "home" to his trailer. It happened so gradually, he just.... fit in. And we'd desensitized our cats to his scent both in their beds, on their brushes, and in their boxes before he ever even came over here. That is the same process you'd use (or that I would recommend) with her in her own room.
So you see, this is why I say it's not about the cats "meeting" each other or "getting to know" each other, it's about them learning to share the space. It's about the existing kitties associating the scent of the new kitty with play, treats, or extra pets (whatever motivates them the most), so they think the new kitty is a GOOD thing (you GOT the kitty just for them!
), and when new kitty is in the same space with your existing kitties, you focus all your attention on your existing kitties, so they have no "extra" reason to associate new kitty with unpleasant things, and they have no reason to be jealous.
It's our natural instinct to be protective of the new cat, and to lavish attention on the new kitty, but this is where things go wrong for people. If you fight that instinct, the space-sharing usually goes much more smoothly.