I had the same thing happen I have 5 yorkies and two non-yorkies. One female (Mandy) started picking on the other female (Krissy) and then the whole pack started picking on Krissy
I tried everything but what worked finally was when they are together and they would gang up on Krissy as the head of the pack I actually get on Krissy side and push and shove the others while growling! It took awhile but she is no longer treated as the omega in the pack. I'm alpha and the rest know there is no room for a second place. I think they know she is under my protection and leave her alone now.
It was really bad at one point I though I would have to get rid of her. Thankfully it all worked out but took time and patience to train them not to single her out and as a pack jump her. I still don't leave them all alone for long periods of time just to be safe.