maybe I'm confused, is that not the same thing as telling Cassie to go to the Lucky 38? Also, if I fire her will I ever be able to get her back if I change my mind?
Having her Wait and having her Part Ways are two different things. You can only have one. When you fire Cass to get Boone, she'll go back to where you found her. If you want her back later, you have to fire Boone and then go to her and re-hire her...just like what you're trying to do with Boone. But since you still have Cass, the speech option isn't coming up for you with Boone. I'd make a new save and then do what you need to do.

You can hire ED-E or Rex to go along with your "human" companion...which includes Raul and Lily too but only one human at a time. Let us know.

If you're on PC, you can get an Unlimited Companions mod and then you can have them all at once.
Added: I think that if you've hired Arcade and triggered the events that will trigger his personal quest (which doesn't happen until near the end of the game) and if you fire Arcade then, his quest doesn't work. If I'm wrong on that, someone can correct me. I've never fired him so can't swear to it.