I have the same situation. 2 smaller, older dogs, and a big puppy. Reggie is a 9 year old schnauzer, and Smokey is a 16 year old poodle. I can't offer much advice, as my dogs still hate eachother and it's been almost 2 months. I'm sure your luck will be a lot better then mine! Smokey really doesn't care one way or the other, he just sleeps all day. He just doesn't like it when Nebo bugs him. Reggie on the other hand.......he growls/bites and Nebo if he even walks by him. I haven't actually had a real "dog fight" or anything, so I hope that doesn't happen. Just don't leave your dogs alone together AT ALL! Supervise everything, and don't let the puppy bug the older dogs for very long.

p.s. CONGRATS on the new addition! I can't wait to see pics! Collie puppies are SO cute!