[mathilda]

Okay, I know some people will have another opinion, but I'd not add another puppy to your family for now.
12 years is already pretty old and the dogs seems to be a bit insecure when it comes to dealing with other dogs.
I don't say it can't work, but it will be stressful for the old dog to have a lively puppy around, especially if it isn't so well socialised.
I'd just enjoy the last years with your elderly dog and think about a new one after the dog isn't anymore.

 

[mathilda]

I think if I would want to add a new dog to an elderly dog, I'd probably take an adult animal that is already a bit calmer and has perhaps also learned a bit about social behaviour before, so that it doesn't pester the old one so much.
there are a lot of middleaged, calm dogs in shelters, that are waiting for someone to take them home.
Teo was relatively fit until the end and very playful, but when he got older he was a lot less tolerant towards puppies and young dog not respecting his boundaries and you could see, after playing with a puppy, that it was tiring him to be with them.