inter-dog aggression

Question
We have 4 dogs.  6y/o altered female retired greyhound, 3 y/o 40lb altered female terrier/boxer mix, 14 y/o female rottweiler/St Bernard mix, and a 1y/o unaltered male boxer puppy.  None of the dogs is aggressive towards people. The greyhound and the boxer mix had lived together (without any other dogs) for 2 years.  We then moved to a home with the other 2 dogs.  They all get along without any conflict.
The greyhound was seriously injured one day by the boxer mix.  The greyhound had jumped against the fence catching her leg in between the fence slats.  She was screaming, hanging on the fence.  All the other dogs rushed to her.  As I lifted her off the fence, she bit my arm.  When she got to the ground, all the dogs mobbed her.  The boxer-mix (who had lived peacefully with her for 2 years) got hold of her neck skin and would not let go.  She wasn't snarling, or growling, just pulling. She seemed unsettled all the next few days since the greyhound was hospitalized.  When the greyhound came home, the boxer-mix seemed very happy - wagging tail, bowing, etc.
No other incidents occurred after that until yesterday.  We had 3 other dogs in the house - my sons dogs who are not aggressive, but they are hyper.  Apparently there was a squabble between the greyhound and one of the other dogs.  The boxer mix again got hold of the greyhound by the neck skin and pulled.  Once she was dislodged, she (boxer-mix)  just stood there looking bewildered.  The other dogs all had to be pulled off.  The greyhound suffered multiple wounds and torn neck skin again.
The boxer mix and I are very attached, I've had her since she was 14 weeks.  She was supposedly abused by her original owners.  She has phobias and is usually very anxious.  When I take her for walks, she gets extremely excited by other dogs, and sometimes will jump and snap at the greyhound as we walk. The greyhound usually ignores her.  They have never had any conflict that I have witnessed.  I am baffled by the boxer-mix's behavior.  I don't think she has EVER shown any aggression toward any dog.  I know that she seems to have a low arousal threshold, and reacts to stress by snapping - but I don't understand the  "pulling" behavior. Her jaws literally had to be pried apart both times.  We will not have any other dogs in the house, as this seemed to be a contributing factor in both incidents.  What else can I do to help her and protect my greyhound from further injury - besides re-home one of them.  She has recently been to the vet for a check-up and is healthy except for some food allergies (which is treated successfully with diet) and reoccurring demodectic mange (brought out by stress.  Her last bout was after the first incident).  The boxer-mix is otherwise very obedient and well behaved - she passed her obedience classes last year with flying colors.

I am absolutely heartbroken and terrified - I love these two dogs.  Any advice would be appreciated.  I am not aware of any canine behavior specialists the this area (Wichita, KS).

Answer
When your greyhound was injured, your Boxer had no idea of the problem; she reacted normally to a situation that alarmed her.  It's not at all uncommon for other dogs in a pack (and even two dogs in a household is a pack) to react with pseudo, or actual, dominance (control) aggression toward an injury or debilitating circumstance in another pack member.  Your Boxer mix is the instigator as obviously temperamentally suited for leadership among your dogs, but "instigator" is really the wrong word: she is doing her job (as she sees it) and the other dogs are following suit.  Bringing anyone else's dogs into your household is a huge error: this disrupts your pack structure and there's no doubt you interact in a different manner with your own dogs due to the presence of the visitors.  Only the most solidly socialized and trained "pack" can tolerate the introduction of even ONE other (new) dog (for however long a period of time, whether an hour or a year); introducing multiple dogs at once is enormously disruptive, and you see the result.

The Boxer restrains and "disciplines" the Greyhound for any perceived fault, but the remainder of your dogs are acting as a "pack" (pack mentality??)  This is a knee jerk reaction and totally normal for multiple dog households where no clear, strong psychological leadership (FROM YOU) is offered.  Her jumping and snapping at the Greyhound is definitely referred aggression and is most likely intended to control the Greyhound in a situation (among strange dogs on walks) which the Boxer construes as problematic.  Regardless of whether or not you have an expert in your immediate geographical location, YOU NEED ONE.  This Greyhound is no doubt living in fear and has already been seriously injured.  If you adopted her from a bona fide greyhound rescue organization, contact them immediately and try to rehome her.  Solving this problem is very complex and requires an in person evaluation and extended consultation, most likely on multiple occasions.  There are veterinary colleges throughout the US and you'd be surprised at how many behavior experts are out there; you have to at least try to find one (Wichita STATE UNIVERSITY has a veterinary college as does Kansas SU.)  A dog being able to pass an "obedience class" means nothing, especially if that class involved coercion.  In order to perceive you as the ultimate authority, the dog must ACQUIESCE to your commands, not be coerced; this involves behavior modification and positive reinforcement training.  You need to become an instant expert in reading dog body language, dog psychology, positive reinforcement training and behavior modification; you need to find a sophisticated veterinarian who understands how to use medication (beta blocker, anti=anxiety (NOT PROZAC or any other SSRI) to facilitate this work.  Even  if you were to read 24/7, I doubt you could obtain this volume of information AND curtail your own natural anxiety in the situation.  FIND AN EXPERT.