I have finches. I breed zebras and bengalese.
I've had trhat problem as well.Have youthought about adding in bengalese (society) finches? They tend to be peaceful and I've found they can greatly calm down a cage. If the dominant pair is just being mean in general and is hurting any other finch or preventing them from feeding, the only things you can do is move their nest lower in the cage (if they have a nest) which will make them slightly less dominant-feeling (it's worked for em on some ocasions) or you can take them out. My agressive pair is actually fine on their own, they are just antisocial. They are WONDERFUL parents though.
I've also noticed that the dark ones seem more agressive. I've only found one of another mutation that was agro as well but he's a silver fawn anyway.

kim