I think there is a rank problem going on here, especially as it is only the bitches.

I think the issue is between Skye and Zoie and the others just join in simply because she starts it.
If Skye isn't spayed, I'd suggest getting her spayed to lower her status even more but leave Zoie as she is if she isn't already spayed and then actively treat Zoie as a higher ranking individual, do everything with her first etc.....
I doubt that Zoie would attack her if she didn't feel any sort of threat. Dogs don't do this for no reason. They only do this when they feel the other dog is a threat to their position etc...
Skye may just be more passive in her approach to climbing the pecking order but it doesn't mean she isn't pushing her luck and the way you treat the dogs could actually be making the problem worse and causing confusion between them.

Next time you witness one of these fights take place, try to remember every last little detail about their behaviour just prior to the aggressive outburst. Nothing is too small and subtle to be of importance so everything from little glances at each other, slight body movements, the lot. It might hold clues as to what the issue is exactly.

Ah, just an edit here. We posted at the same time so now I know they are all fixed.

As a rule in a multi-dog household, you need to treat them strictly according to rank. I think because they've all suddenly been lumped together and probably because you don't already have a rank routine in place and just treat them all as equals, they are confused as to who is higher ranking than the other. They haven't managed to establish a clear cut pecking order so thats your aim in order to restore peace amongst them.
You need to figure out who should be naturally where in the pecking order by carefully observing their behaviour and then use this info to decide who is where and treat them that way but you must ensure you are higher ranking than all of them at the same time.