We got a Lab puppy and a mature kelpie x something (Doberman or Rottie perhaps) from the pound about 10 years ago. They have been great friends to us. Now we've got a young family, and they have not missed a beat. They are both very good with the kids.
Labs are popular for a good reason. They are so placid and even tempered. I've never seen Lenny show any aggression towards another person or animal in his ten years. He can be boisterous with other dogs, who may snarl or bare their teeth at him. He doesn't understand what is going on and thinks that they are playing. Maybe he's just not that smart.
I was hesitating about getting a kelpie due to their energetic reputation, but Zimmy was a mature dog when we got her who was already well trained and not the last bit hyper-active. She's actually quite sedate, and like all Kelpies, super smart.
Lenny and Zimmy the week we got them (2007)
Lenny and Zimmy this morning