I would suggest not getting a new cat, as your current one obviously doesn't like other cats and since he is a rescue cat, you have no idea what happened in the first 10years of his life and this may be a deep rooted problem not just a teritorial one.
However if your insistant on wanting a new cat, I would recommend a kitten as they are less threating then bringing in an adult. To help your cat adjust always praise and treat him when he and the kitten are in a room together, if he associates the kitten with getting more positive attention from you then when he's with you on his own, he will see it as a good thing to have the kitten around.
Also if possible give him and the kitten a bath when you first get it (seperatley of course) this will help remove any poutsider smells from the kitten, plus it will give both of them a similar smell.
Make sure the older cat is fed 1st and given attentrion first etc, before showing the youngster attention so your older cat still feels he's in charge. As a kitten the other one will not be as bothered about being 2nd as an older cat would be. Above all make sure each has its own bed, food, toys etc and at the start they are not forced to be together, however this lot still might not help if hes behavior is a deep rooted problem instead of just teritorial