My Pouncer used to really hiss and spit at Mr Scrappy to the point that I thought I might have to take Mr Scrappy to the SPCA.
I actually had him in a carrier and he started to cry.
Then I let him out and Pouncer came over and started washing him, and they were Best Pals after that!!!

Patience ,Patience and More Patience.
Love will find a way!!