Okay, I'm sort of an advocate for Liza here... IMO, the point here is to get the cats to all live together. If it doesn't work, one or more of the cats are going to get rehomed, and honestly, while it's not the opinion or the choice that other people here would make, ultimately what it comes down to is ... if she can't make it work then she's going to have to rehome cats. Period. Some kitty, no less deserving than another kitty, will have to find a new home. I also don't think it has anything to do with the breed.. she stated early on that it has to do with who is being the aggressor.
I don't think you need to talk to the vet about it more than you already have, unless it's to discuss medications. He's not going to be able to help with behavior. IMO, you need to talk to a behaviorist. Since they are slightly expensive, I'd personally try putting some of those soft paws on the attacking kitties, to avoid injuries to the newbies.. and then I'd probably try to use some of Kerry's techniques. A lot of similar situations to yours were covered in the thread I linked in earlier on.
One of the solutions offered there was to split up the two fighting kitties, not to scold, yell or otherwise give them any attention, positive or negative.. then put them in separate rooms for half an hour. There was another solution offered as well.. to me, if you want to look for it under my post in that thread.
There is also this, although I'm not certain whether it will help or not as it's about kitty aggression to an owner, but it does involve other animals, and it might be useful. http://www.thecatbehaviorist.com/
Click on the aggression tab.