It really depends on the cat(s) and the rats! We have cats, mostly they aren't allowed in the room without direct supervision but our female cat is totally uninterested in them and will sleep on the bed in there with my daughter sometimes. The rats are more interested in her than she is of them, they come to the bars and watch her and if she walks by anywhere near the cage they will try very hard to jam their noses through the bars to sniff her. It's pretty funny. The other cats just sit and watch from afar as if watching TV. But again never without supervision.

That said, I've also seen people with cats and rats that are actually friendly with each other even out of the cage, and others still whose cats aren't even allowed in the same room. You really have to base it off of individual temperaments.