We have a 6 year old neutered male. Generally pretty chilled out, not a fighter. His breed is quite dog like and keen on company so we thought he might appreciate another cat, especially if we're away.
Picked up a 1 year old rescue female just over two weeks ago who was living happily with a younger male in her foster home. She has had her own space with separate feeding and toilet area and we did the introduction slowly after a few days - scent swapping, feeding at door etc. Whenever she sees old cat she's instantly aggressive - hissing, stalking him, swiping at him and blocking his way through doors etc. She'll go right up to him just to hiss in his face so it doesn't seem like she's particularly scared. We're not leaving them alone together and are separating them after these incidents but it's making old cat very stressed. I know it's early days but is it possible this just won't work? Have had multiple cats in the past and introductions were never this bad!