I recently introduced not one but two new rats to my mischief. I never felt that term for a group of rats was more appropriate than when I went through the motions of introducing the new  babies to the older girls.
As usual, Kiri was very accepting of the newcomers. Kiri's accepting of almost everything (except for a thermometer in her behind at the vets, wasn't that a show!), however Ezri was most certainly not, and Dax followed her lead.
Ezri wasn't well at the time, she'd recently had a myco flare up. She's also fairly opposed to change in the cage. It took several attempts at introducing them in a neutral space before I tried to place the new babies into the main cage. They were met with the disapproval of Ezri, swiftly.
I was almost ready to resign myself to the  fact that the new girls would have to live in one half of the cage and the old girls in the other, but then I remembered something.
I put Ezri and Dax in the smaller cage for a few days, letting Audrey and Ro live in the larger cage with Kiri. I then changed all the hammocks and liners and cleaned the cage with perfumed wipes. Once I tried introducing them again, it went well!

They now all live happily together. I really recommend trying the last method I used if you're introducing rats to your colony. Making the cage seem completely different in how it's arranged and how it smells makes all the difference. It turns it into neutral territory that's comfortable for them. As you can see, through this my girls have achieved cage harmony!

I have recently welcomed a new addition to my mischief, Ro. She was adopted from a pet shop (terrible of me, but I couldn't help it!) after I watched her over the week all by herself in the cage. I brought her home and I'm glad for it :)

She has a beautiful temperament and a lovely coat. I believe she's a Cinnamon Rex (as if I needed more Cinnamon babies... haha). She was accepted fairly quickly by the other girls, and now happily snuggles with them.

I introduced them by placing them all in a neutral area (the couch) and allowing them to interact. After half an hour it was going really well, and seeing as my girls are often happy to share space with any old random rat in their cage, I put them all in there to watch their behaviour. Dax couldn't care less and Kiri adopted her as her own, but Ezri wasn't a fan of sharing space with Ro. So, Kiri and Ro went into the travel cage for the night to be re-introduced the next morning, which went really well, and every one's been happy since.

Welcome home, Ro!