Roommate wants "girlfriend" to move in

Brandie
Here it goes.. me, my boyfriend and his friend decidedly to move out together. We were all living together for a few months and then decided to move out to a new place that takes pets. We had to move out of our old place asap because we need to move out with our pets which weren't accepted with the landlord. So I didn't want to move out with the roommate to begin with but we decided to ask him to move in with us because it would be cheaper and the roommate doesn't know anyone from where we live(he's from up north and we live down south). So we moved in in November. I put the deposit down and told the roommate to pay me back whenever he can. He has paid me a little under half of what he owes which is not a big deal. Me and bf do all the cleaning, buy everything for the apt(like paper towels, water, couches, fridge, etc. Literally everything in the apt is ours except what's in his room) and when there's a problem with the apartment my bf is the one to talk to the manager even if it's the roommate that has a problem with the apt. My bf thinks of him as a little brother. He's 26 I think and my bf is 33. The apt basically feels like just ours since he's never home(he works night shifts and sleeps during the day). He's a cool guy. Super awkward tho. Just constantly stares and doesn't say thing. And is CONSTANTLY leaving lights on all over the house. He's super forgetful. He's a good person and a good roommate but I just really hate living with someone other than my boyfriend. It's nothing against him, I just can't stand not being able to walk around without a bra and stuff like that lol i just like my space. And I have told my boyfriend this numerous times and we have decided that we are gonna move out just the two of us when the lease is up(this November). Any who, our roommate had his ex girlfriend visit this past weekend and we hung out with them on Friday. I've met her before and she's a cool person. I like her. My bf knows has known her for at least 5 years. When we were with them, it just seemed as if they were friends. He were barely close to each other and rarely sat next to each other. I think I barely saw them talk. So I thought it was weird that she would take time off work and travel down here when he seemed to awkward but hey whatever not my relationship. So finally the climax: our roommate text my boyfriend today and said: "____(his ex gf) is moving in". My bf: "good one"(thought it was a joke). Roommate: "I'm serious. We were talking about it and decided to do it". My bf: "when". Roommate: "when she finds a job out here". My bf: "we will talk about it tomorrow" 
Just last week they weren't even together and now they're gonna move in together? Also, they have never lived together. Seriously I'm so annoyed. Of course our rent will be cheaper. But I really don't care about that. Im starting my business from home and I just can't stand the thought of having another roommate. I have expressed this to my bf and he doesn't want to have her there either but we know that the apt is equally his too so I don't know how much say we have is this. My bf keeps telling me that it probably isn't gonna happen but I have a gut feeling that it is. The only way out of this not happening is if she wants to bring her 2 dogs. We already have 2 here and 1 cat and my bf said she can't leave here if she wants to bring them. Our apartment is very small also so having 4 people + 4 dogs + 1 cat is ridiculous. Plus this apt is already my dogs territory. I can't even imagine the chaos that will happen. What would you guys do?