Lovely dogs! I need to find someone who’ll lend me a large, placid dog for a weekend to see how my too-many cats will react. I want a collie, but I’d like to gauge the level of unhappiness my cats will feel before making that commitment.
You really should try your cats out on a puppy. They will be totally intimidated by a large grown dog, and the dog might have an opinion about cats too. The cats will control the puppy and the younger cats will even end up playing with it.
The Rough collies (the 'Lassie' dogs) are calmer, but any young collie might tend to agitate the cats -- since they all have herding and nipping tendencies. An older, calmer dog would probably be better for the kitties than getting a puppy. Just about all puppies are more active, curious, and mouthy than a grown dog of the same breed. And then of course you wouldn't have to go through housebreaking!
You might get somebody to loan you an older collie (who knows the command "leave it!") Or you could go to a Collie Rescue organization and ask for one that is cat safe. They might even bring one to your house and let you see how the cats react. Our rescue people here locally are very flexible and work hard to find homes for their dogs. They foster out their dogs before they place them and have a good idea of their temperament and habits.