Dogs are wonderfully adaptable creatures. Watching friendly dogs interact with each other is always entertaining. And the beauty of the matter is that just about any properly socialized dog can get along with just about any other properly socialized dog. Chihuahuas can play and live with Great Danes. Puppies can rejuvenate grouchy old curmudgeons.
If you have raised your Jack Russell properly, he will get along with any new dog that you adopt. And I have a long-time saying that I think applies to your situation: don’t get a pet for your pet. Get a pet for you.
In other words, you should adopt the dog that you want. Choose the age, breed, gender, and provenance based upon what you want, not what you think your dog will want. I am confident that the dog you choose ultimately will get along with the dog you already have.