Of course some dogs prefer to live alone!
You could probably train or manage them to get along (either through reinforcing good interactions or a crate-and-rotate schedule), but if you don’t think it’s fair to your min pin, then don’t do it. If your dog is a senior and you think a puppy would cause some serious disruption and tension, it seems nicer to let your min pin live out the rest of her life as a single dog, and then get the puppy in a little while after she’s gone.
It’s hard to convince spouses to put their desires on hold, especially if they’ve wanted a puppy for a long time, but I would just explain that you don’t think it’s fair to bring a puppy into a household with a single senior dog, when the senior dog prefers being alone. You could also mention that you (human) might end up resenting the puppy for the stress it causes your senior dog, and how that’s not fair to anyone involved and you would prefer to wait before getting a puppy.
I would definitely try to spin it as “it’s not fair to senior dog or to new puppy” more than “it can’t be trained”. Hopefully your spouse will be more open to waiting in that case. Good luck! -C