Give them some time to get used to one another. It may take quite a
while, (more than a month), but eventually they should start to at least
tolerate each other.

Time is all it takes. I foster cats; one of my adoptions placed a cat
with two Yorkies (we'd tested the cat with the Yorkies, and she wasn't
afraid of them, although she hissed when they went too near.)

The adopter had a house on several levels and made sure that the
cat had a "safe" place where she could avoid the dogs. Things
happened gradually: first the cat found a windowsill out of the dogs'
reach where she could lie in the sun. Then she started to go into
the same room with the dogs, still hissing if they got too close.
(The dogs were panting with enthusiasm through all this, by the
way!)
Then she was seen touching noses with the dogs. The adopter sent
me an email a couple of weeks later telling me that she'd found the
cat and one of the dogs sleeping together on her bed! Since then
she has sent me photographs of the cat sitting happily beside the
dogs (the dogs look like her humble servants, actually.)

Marion

I think you should just give it time. Make sure the cats have a place they
can go to that the pin can not go. this way they will not stress completely
out. Really by the time you stop worrying they will be used to each other
and even possibly playing.

Shadow Walker

it will take lots of time try looking at this link
http://www.carepets.org/catsub/cat_pdf/intro_catdog.pdf