When I first got Vinnie(12 week old foundling),I almost gave up and it takes a lot for me..
Trying to get my other cats to accept him,my nerves were frayed and I cried a lot...but it did eventually work out
My one cat(Chico)still does not like Vinnie,but theres no fighting.
Vinnie and Rocky are best buds and often playfight,no hissing or growling though.
Sometimes Vinnie will work up the courage to chase Chico,but one hiss from Chico and Vinnie backs up
Chico is the Alpha cat,for sure!!
So please don't give up,it will get better