I have a two year old male rat named Nico and he has slight hind leg degeneration, so he is more of a lap rat than running around rat. We recently got another rat from our local PetSmart, about 3- 4 months old named Remi who is completely healthy. We had him in quarantine for about 3 weeks and then started the first part of the introduction process- putting the cages near each other. Thy seemed to be interested in each other, so we did it about 4 more times before moving on to the next step, introducing them in a neutral territory. We chose the bathtub and put some food and a tissue box inside of it. When we put the rats in it went pretty well for about ten minutes, until Nico, (The older one) tried to climb into the tissue box that Remi was occupying. Remi just jumped on Nico and when we got them apart we found that Remi had puled a clump of fur from Nico's shoulder. We kept them separate for a day or two before trying again. This time we did not put any tissue boxes in and rubbed them both with vanilla extract. It went fine for some time until Remi randomly stated scratching at Nico's shoulders really vigorously and we separated them. The next time we tried it the got along for the longest time and then Remi pinned Nico down and started power grooming him, and literally flipped Nico over on his back. I 've read about rats pinning down and the submissive rat letting out squeaks, which is what happened, but it seemed pretty fierce. Was this okay? And if so, how do I tell when they're ready to live in the same cage?