Awww Shannon...I'm sorry that things aren't going perfectly smooth for y'all. You are such a dog lover and I wish things could be nice and smooth for you and Boomer and Cassie but...reality stinks I'm sure it's RARE for people to bring home a puppy to an older dog (especially a dalmation) and have it go well at first. Moon and Bean are still hissing and slapping at Sunny but we're getting there...slowly. Here's my thought and Sunny is my first puppy so this is purely just a hunch: I think you need to find some older dogs who ARE good with puppies (maybe talk to people at petsmart; are you going to stay in touch with any of the people from Boomer's puppy kindergarten class...do they have older dogs too who are now used to a puppy?) put up a notice in petsmart "looking for puppy/dog playdates" or try to find a dog park) etc... Maybe if Boomer gets used to playing with other older dogs, he will learn how to heed their warnings and may get better at "playing" with Cassie. I can imagine how hard it must be for Cassie that Boomer is taking out all of his puppy energy on poor Cassie. You have to give him an outlet for that somewhere else.
Good luck/ it WILL all work out