No, baby lambs do not belong with baby puppies! Cute? Yes. A responsible choice, no.
Think of the first livestock as mentors for your pups. One of the ways to use early livestock is to pattern for the pups behaviors you want to see more of.
Pups will play off of the energy of their livestock during this impressionable time. If you want your pups, as adults, to be effervescent fools, give them lambs! Nothing is as simple as black and white, and some pups can handle this, but it wouldn’t be my choice.
If you want predictable, slow, and steady, gentle LGDs, give them slow and steady and gentle livestock right from the start. Often the first LGDs to you are the first to your livestock as well. Give them space! Allow them to share a fence line with the pups right from the beginning. The pups will feel connected, the livestock will feel safe enough to become inquisitive, and the magic will begin to happen.
Watching all this develop right in front of you can be mind-blowing, or at least that is true for a training geek like me. To set this physical situation in place, go to sleep with the memory of wide-eyed sheep and uneasy pups, and go out in the morning to find a sheep and a pup sleeping against the fence as closely as they can be to each other is amazing.
We are only stewards to these animals. Their relationship is complex, and I don’t begin to pretend to understand it. I just tweak the environment as best I can to facilitate the relationships I hope to cultivate and then settle back to watch the language develop.