I'm guessing the pictures are of KitKat? She's absolutely gorgeous
Primarily, has KitKat been throughly checked over by the vet since you brought her in? Is she spayed?
Did you follow the introduction process to allow slow integration into the household - initial isolation, exchanging scents (swapping of blankets to let the others smell her and vice versa/damp towel rubbed on the chin, shoulder blades and base of tail), supervised meetings, Feliway?
If not, don't worry, you can always start the reintroduction process again if that's what you choose to do.
There's a really good sticky at the top of this forum with instructions.
From my experience, It can take a fair while for cats to adjust to each other. The original cats feel as though their territory is being invaded and the hierarchy is upset with the new addition and it's possible that the new addition hasn't been accustomed to living or interacting with other cats.
When my Willow arrived, he had no idea how to react around Tibby and Molly, choosing to spend his time with me. Although they all got on pretty well for a while, Molly decided that her position in the pecking order was being jeopardised and began a power struggle with Willow - which often led to Tibby joining in. The group could change by the day and the 'bullied' varied greatly.
Feliway and Vanilla Extract (dabbed on all cats on their scent regions), under the chin, base of tail and between the shoulder blades helped a lot. But we did need a lot of understanding and patience, which admittedly can be rather trying at times
I also provided a great deal of hiding spaces, high spaces, feeding areas and litter trays to ensure that nobody felt they lacked their own territory or were being bullied away from fundamental utilities.
Things aren't plain sailing here, but there has been a vast improvement and we continue to work at it, learning together!
I think that if you are financially and emotionally able to give this a go, it would be well worth a try
Two months isn't a very long time to expect such complex creatures to adjust
ETA: Often, shooing isn't an effective method as it's possible to make the cats more angry at each other. If the brawls are getting very heated with teeth, claws and fur flying, throw a heavy blanket over the fighting cats and remove the aggressor from the situation, checking both for any injuries.