I too am having a bit of a problem with my 9 month old puppy Jake and two other dogs. Lacey is almost 8 and Layla is 2. They are so much bigger than Jake and it seems Jake doesn't heed their warnings when they don't want to play. They are always supervised and I'm sure you already do that. I too find myself getting nervous when one of them warns the puppy, like "is she going to bite him?" We are trying to train Jake to leave the girls alone when they warn and the girls are told "don't you dare" while I get him away from them. Lacey is very good at the "don't you dare" command, Layla is getting there.
My best advice is to wear the puppy out as much as you can and teach Boomer to respect Cassie's warnings. Hopefully, in time, when Boomer settles down a bit, they'll get to be friends.