Sometimes people will say things like it’s better to introduce a new animal to a younger pet because they can tolerate the stress, or it is better to introduce an animal of the same sex. The truth is every animal is different, and until you bring the new animal into your household, no one can predict how your animal will react. For this reason, it is hard to make recommendations based on age/sex of existing pet and age/sex of new pet. The most important thing to keep in mind is that there will be a period of transition while your pet gets used to a new animal. If they do not get along for the first several days, do not be discouraged.
1. The first contact between animals should be underneath a door. Confine the new pet to a small room such as a bathroom, and let your animal investigate smell/sound underneath the door. When your animal seems more comfortable with the idea of a new pet, you are ready for the next step.
2. Place the new pet in a crate and place the crate in a room where your animal can come and investigate the sight and smell of your new pet. You’ll have to be in the room to supervise. You may let them sniff, etc. for however long you will be in the room, but when you leave, take the new pet with you and isolate the two again. It will take several days of supervised investigation through the crate before you can move on to the next step.
3. Put one animal on a leash, and let them meet face to face without the crate. The leash will give you control of the situation should there be a fight.