Ohio State University has started an indoor pet initiative. On their website there is great information on how to train your cats and dogs and what to do about stressful situations. In that cat section they give advice on how to keep all the cats in a multi-cat household calm. For dogs they have a section on how to train your puppy and what having a puppy entails. They also have great, reputable links to other websites.
Click here to go to their Indoor Pet Initiative Page!