Thanks for the replies. Like Michelle said-we talked on the phone and I'm going to do some work on this. Seperating the yard may not be possible-Mom is a huge obstacle over here-I will discuss this with her. I also think she is part of the problem-she does not make the dogs do anything, is very passive-a lot of the bullying happens when I am not home and she is.

They are never totally unsupervised-Skye, Romeo, and Star are crated when we are not home. If they are out in the back, the slider door to the porch is usually opened and we can see everything going on through a big window. I do realize it only takes a second for something to happen, even with supervision. For now, most of the time, Skye goes outside seperately until the situation is fixed.

To answer some of the questions-all the dogs are fixed.
If they all go out together-here's one scenario...they will go pee or whatever...then start wandering around..then maybe zoie will go up to her and start mouthing and bumping into her like she's wanting to play. They might start wrestling and then another dog will get into it...and then another..and all of a sudden it has escalated from the wrestling into all dogs on one. And then there is times when she will just be coming to the door, and zoie wil jump on her and then the rest follows. There is a cement bench by the back door, and usually if she can get onto that she can avoid any confrontation, for whatever reason. A lot of times they won't let her get this far and keep her down on the ground.
She is submissive and not very confident. She does her submissive roll on her back and keeps getting picked on. We are working on building confidence but it takes time.

So thanks guys for the responses-I will keep you updated.