We have a 7(ish) yr old Tom cat who came to us as a stray, which is why we can only estimate his age. He's always been quite nervy, strongly territorial and semi feral.
We brought Blue home two weeks ago, and the introduction has not gone brilliantly. The cat is afraid of Blue and avoids him wherever possible. Blue is curious about the cat and would love to get near enough for a sniff and potentially a nip. There's fat chance of that happening, as he has barked at the cat whenever he had an opportunity. The cat is occasionally deliberately provocative, and if it came to a scrap, my money would be on the cat, but so far we are just accepting that they'll not become friends anytime soon and the cat has safe spaces within the house which Blue can't access. I'm using both Adaptil and Feliway diffusers, and Blue gets a reward if he sits near the cat without barking or attempting to pounce. It's not ideal, but better than the expected outcome, which was that the cat would just walk out and never be seen again.