You can introduce them after that, but it may take a couple of weeks for them to accept each other. For more information about rats, read the Pet Rat Information Sheet . It is the draft of a book on rat care, with the aim of helping people to keep their rats happy as well as healthy. I am biased about this, being one of the co-authors. There are some wonderful ratty websites.
Gerbils are like little clowns - happy, cheerful and bouncy.
Gerbils produce very little smell, and don't need cleaning out as often as some other animals. They are easiest to keep in glass tanks as they will chew their way out of anything else. Try keeping gerbils or jirds in tanks full of compost - they will enjoy burrowing and making tunnels, and it will keep their coats smooth and glossy. Find out more about them from the UK National Gerbil Society homepage . If you want something more exotic, Shaw's Jirds are like giant gerbils with similar habits and requirements.