Older Alpha Dog not tolerating New puppy

We have 2 older dogs, a 7 yr old sheltie and a 4 yr old border collie mix. The sheltie has always kind of been alpha and the border collie submits and is cool with it. They aren’t best pals, but they get along fine We just brought home the Newfie puppy about 5 days ago. We have maintained that the sheltie is alpha by feeding first, greeting first, and keeping his toys away from the puppy. He also is the only one allowed to sit with us on furniture. The puppy is naturally rambunctious and bounces and paws at the dogs. The sheltie is NOT having it. He stiffens and tries to turn away a bit, but doesn’t actually move away and eventually will snap. The puppy cries loudly and drops to the floor. I haven’t seen an injury yet from the 3 times this has been the result, so the puppy may just be sounding very dramatically out of fear and communicating submission. But it still concerns me. I don’t even expect them to be best buds. But I do expect them to tolerate each other eventually. The 4 yr old is doing great. He doesn’t really want to play with the puppy, but he moves away and walks away, and occasionally growls as a warning without snapping. I really am trying to protect the senior dogs from the rowdy puppy behavior, but I know this rowdy stage could last a while and the puppy being a Newfie, he will only get bigger and stronger. I separate them, but the sheltie tends to be jealous and if I’m paying any attention to the puppy, he wants to be part of it too…but then gets annoyed when the puppy plays with him. Are there any other tips you can give me to help the older dog to tolerate the puppy’s behavior a bit more.