I *mostly* successfully integrated a solo ferret into a bonded pair family. When they first saw each other the solo guy started screaming and hissing. After months of gradual interaction(from separate cages and play times, to forced together bathtimes, to shared play time only to shared cage and occasionally separating them when they fight) they were calmer and calmer together. Now they sleep in a pile all together. The previously solo guy still doesn't quite seem so bonded as my original pair but everything is fine. So I think first impressions isn't the be all and end all.
At least you have a wonderful ending to your story! We just introduced mr noodles to Rico almost 2 weeks ago now. You would have never guessed Rico was an aggressive ferret.