agreesive female aussie towards dogs

Question
We rescued a blue Merle Aussie in April of this year that was dropped off at the pound.  I have raised several aussies in the past. She had been a breeding female at a puppy mill. She got kennel cough at the pound and was going to be put down that day if someone didn't take her. It took nearly 4 weeks before we could bring her into our home with our other dog a Queensland blue tick heeler only 4 months old. Since I imagine she had been kenneled most of her life she didn't even run.  We have two acres that she can be totally free on, but it took a couple of montHs for her to chaser a ball.  In the beginning she took all of the puppies toys away from him and barked at him if he was to come out of the bedroom.  Eventually, that all changed and they became good friends, she ran, played, even dove in the pool, she actually  can swim UNDERWATER!

In May we inherited a 13 year old Boston terrier, after the passing of my mother in law.  This didn't seem to have any effect on either dog.  In August my husbands daughter moved in with her black lab mix (exceptionally well mannered dog-better than most people) I refer to him as the mediator.

All had been good, with one exception around Memorial Day. My parents came with their small female poodle who is quite snappy and really gets along with no one. My mother has a complex as she says my dog doesn't even like me... and that dog is spoiled rotten
But the poodle snapped at my Aussie just as she was walking by her and my Aussie turned around and grabbed her by the nap of her neck and would not let go.  She had never showed any aggression like this before at all or sense until this past week.  

I had been a bit nervous if people brought small dogs "puppy" size over as I thought perhaps she was sick and tired of whining puppies.  However, she didn't even pay any attention to anyone else's dogs, but they didn't snap at her either.  This week, for no reason at all she lunged at the Queens;and blue tick heeler and held her down biting at him and fighting and he was just crying in pain and I intervened and pulled her off as the other one ran.  I could only hold her so long and when I lost my grip she took off after him again.  Thank goodness I had others in the house to assist. I picked all 45 lbs of the Aussie up and put her in the kennel which is never closed, but this time I locked the doors and left her there form 30 minutes. Then I let her come outside with me alone for about 30 minutes and played ball and everything was normal.  We went inside with the other dogs and she went straight for the heeler again and attacked. That was the worse day. Throughout the week she has been getting a bit better.  Yesterday she attacked him, but I yelled at her to stop[ and she stopped instantly.  But my poor Queensland Blue tick heeler is so afraid he is either right by myside or under the bed, behind the couch, or searching for a safe haven.

I did read something regarding thyroid issues could cause this type of sudden aggression.  She doesn't act this way towards people at all.

Any thoughts,  She is beautiful and very loving, but possibly quite jealous and needs lots of attention.

Answer
This is a situation that should be addressed directly by a trainer who can work with you.  On a couple of fronts, I do have some temporary advice, though.  The first thing any competent trainer does when they want to reduce an unwanted behavior (in this case, the fighting) is to find a way to stop the dogs from rehearsing it.  So, I would, temporarily separate the dogs that are fighting.  Since you don't know your dog's history, I would definitely have a thyroid test done (your vet should send the sample to Michigan or Hemopet).  Sudden behavior change in an adult dog is ALWAYS a reason to seek help from your vet.  Just be aware that it is also possible that there is another reason for her behavior.  Some Aussies can be possessive of toys, food, owners, space, etc. (the joke is that they are Bosstralian Shepherds) and that can lead to tiffs.  Others are only aggressive toward members of the same gender, or dogs that are a certain size.  Speaking of size, if you are at all worried, do not let her interact with dogs that she outweighs by more than half.  So, her playmates, if any, should weigh more than 22 pounds.
The good news is that no one really bit down on anyone to cause wounds.  I classify that as a "tiff" not an attack or fight, and the dogs seem to have good bite inhibition, which is actually a predictor of success if you decide to hire a trainer and work with your dog.  Try to find someone who is experienced with aggression and uses clicker training or positive training.  Aggression is often made worse by correction, even if the initial results look promising.  Pet Professional Guild, Animal Behavior Society (behaviorists), Karen Pryor Academy, Academy for Dog Trainers, all have listings.