Hi Prodigal,
I am so, so glad to hear that you are taking care of animals that need help. This is such a problem in most cities. Kudos to you for doing that.
I have three cats, all male. I have twins and one who is a year older. It took a lot of work and many weeks to get the older cat to stop hissing at the babies. But, now they are the best of friends.
I wanted to suggest that you speak with Phinneas. She is, in my opinion, the SR expert on this kind of thing. She has lots of experience with rescued animals and had many good tips for me, when I was dealing with the hissing crisis.