How can I get my cats to get along?

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We have 7 indoor cats. Two are female and the rest are males. OUr oldest female only likes the male cat when we got them together as babies. The other cats all get along, but the oldest female is especially mean to the other female cat who is about 6 months old. We have tried squirting her with a water bottle when she gets mean, have stuck her under the faucet and have even locked her in the pet taxi for a few minutes when she gets like this. Nothing is working and the vet said to give it time. But I feel sorry for our youngest female. She is clearly scared of her. Giving one up is not an option. Any ideas?