We just added our third Chihuahua back in August. Finley came to us as a 7 week old puppy. After we had told the breeder we would take him, and were waiting for him to come home (we'd been talking to the breeder about another pup before the litter was born, so he was ours right as soon as he was born and we had to wait FOREVER to get him lol), I had some very serious second thoughts about taking in a new puppy. We already had our two adults - Tinkerbell is 11 years old and Gizmo is two. They absolutely adore each other, and we've never had three dogs together before, so I was very very concerned about disrupting the dynamics of our household and of the two existing dogs. Now, three months later, I don't know what I wasted so much time worrying about. Things could not have worked out better, and Finley has been a wonderful addition to our family.
Before our existing three, we had another Chihuahua named Shadow. We adopted her as an abused rescue from a puppy mill. Tinkerbell was a little over a year old at the time we brought Shadow home, and Shadow was about 5-6 years old. Shadow was the sweetest dog on the planet, despite the horrible abuse she had endured, and she was with us until she was 14 years old. She never got cranky or short tempered at any point in her life, right to the end. What's more, I fully believe keeping a younger dog with an older dog has some major health benefits to the older dog - a younger BFF helps keep them younger, too!