Breeders, keepers, importers and lovers of Havanese dogs
Although the dogs must develop their own social order, they must remember that you are still in charge. This means being fair on reprimands as well as affection. Any misbehaviour, regardless of who started it, must be handled equally (and with consistency). When play gets too rough, all must be separated until calm is restored.
For the first few weeks, keep an eye on the dogs in situations that might trigger aggression, such as when you come home, when guests come over, going out to the backyard, preparing to go for a walk, mealtime (theirs and yours), and playtime. It is very important that you spend time with each dog alone so that the resident dog continues to receive one-on-one attention and the new dog develops a bond with you.
Reward your existing dog for her calm behaviour in the puppy's presence. Never physically punish her for reacting with a growl or snap. As long as she doesn't actually harm the puppy, she should be allowed to set boundaries by growling and even snapping. If she doesn't correct the puppy for rude behaviour, you have to step in and correct the puppy with a stern "no."
It is best to prepare for the introduction with a barrier such as a crate to be used in the initial introduction process.
When you arrive home, allow your puppy to first go to the toilet in front of the house. Bring the puppy inside in the crate and place in the middle of a room allowing your existing dog to sniff and bark at the new addition. Leave the puppy in the crate until the existing dog has lost interest (usually takes no longer than half an hour), then take the crate outside and open it for the dogs to meet face to face.
When the dogs meet face to face, one of them is going to establish dominance. This is a normal and necessary step in a dog-dog relationship. The dogs will maneuverer around each other and may even scuffle to the point at which one dog ends up on his back, with the other dog standing over him. There may be some nipping and grabbing of the neck or throat. Try not to worry too much when this happens. It is normal for dogs to engage in such roughness. A scuffle is a dog trash-talk argument and are brief.
Once the dogs are playing well together you can bring them inside and keep an eye on the dogs by watching their body language. Do not leave the dogs alone together for the first few days until you are sure they are confinable with each other.