Your concerns are very valid. It is entirely possible for a large breed puppy who doesn't know his own size or strength to accidentally hurt a small breed, elderly dog.
Keeping them separated is the only real way to completely prevent injury or stressing your elderly dog, I'm afraid. Allowing the puppy to see but not play with the older dog helps him get over the novelty of another dog, but it does take time for puppies to learn manners. You can achieve this using an exercise pen or baby gates. You could also try having the pup drag a leash (supervised, of course), and when the pup seems to want to bother the older dog, you gently lead him away. I would only try this once the pup was not losing his mind at the presence of the older dog behind the barrier, though. You Chi clearly does not want to play with the puppy...so don't make her. Prevent the puppy from doing so. It sounds like she just wants to left alone.
In a nutshell, there is no easy overnight fix for this. All solutions will involve time and separation. The setup is only temporary, but it may last months, perhaps a year or more depending on how determined your pup is to play with your senior.
If this is not something you can manage, I would suggest rehoming the pup. I would not get another dog until your senior Chihuahua passes since she does not care for other dogs, or you may end up in the exact same situation even if you opt for a small breed dog or an adult dog. This will make her last years with you as stress free and enjoyable as possible.
Keeping them separated is the only real way to completely prevent injury or stressing your elderly dog, I'm afraid. Allowing the puppy to see but not play with the older dog helps him get over the novelty of another dog, but it does take time for puppies to learn manners. You can achieve this using an exercise pen or baby gates. You could also try having the pup drag a leash (supervised, of course), and when the pup seems to want to bother the older dog, you gently lead him away. I would only try this once the pup was not losing his mind at the presence of the older dog behind the barrier, though. You Chi clearly does not want to play with the puppy...so don't make her. Prevent the puppy from doing so. It sounds like she just wants to left alone.
In a nutshell, there is no easy overnight fix for this. All solutions will involve time and separation. The setup is only temporary, but it may last months, perhaps a year or more depending on how determined your pup is to play with your senior.
If this is not something you can manage, I would suggest rehoming the pup. I would not get another dog until your senior Chihuahua passes since she does not care for other dogs, or you may end up in the exact same situation even if you opt for a small breed dog or an adult dog. This will make her last years with you as stress free and enjoyable as possible.
