My boyfriend's brother moved back home a year or two ago and took his own two cats.. Matt's dad already has two cats. The 2 new cats had to live down in the basement (it's done up like an apartment) and the two established cats lived upstairs.. Matt's dad would go back and forth smelling like whichever part of the house and the cats got used to each other's smell like that for months. And since Matt's dad was "alpha" cat (in whatever hierarchy cats have) the cats somewhat respected him smelling like the other cats.. It's hard to explain but that's how it was for months. Then one day he got fed up and just opened the door separating the cats and walked away. The cats tolerate each other lol.
There's a happy cat diffuser you can buy and plug into the wall which might calm them down.