I agree with Kay, it depends on the dog. I've had two females, Lacey and RB Mandy, now Lacey and Layla. Basically, they got/get along fine. Then there came Jake, our little boy. That brings me to size. Lacey's about 80 lbs,Layla 70lbs. Jake is about 20lbs and I don't think he'll get a lot bigger. They all play together fine. Sorry, can't help you out with adopting a dog/older dog versus puppy, mine were all puppies.