All of my cats grew up around the rats. They play and eat together, groom eachother etc. I would never leave Onyx unattended with one of my small animals though, as he grew up feral. He's very good with them, and eventually he will get used to it, as all my cats have. Rats are VERY smart, and if they know they are in trouble will defend themselves. Nemesis could very easily hurt Onyx if he wanted to. Don't underestimate the ratties.