Why does my cat hiss so much?

Cats hiss because when they’re frightened/stressed and there can be both medical and behavioral causes of this. If Shoes show no sign of any physical problems—i.e., he eats and drinks normally, no vomiting or diarrhea, runny eyes or nose , or any other symptoms that would suggest a medical problem, then we can say his stress is behavioral in origin. You don’t mention how old Shoes is, but cats who were taken away from their mothers prematurely (usually before 8 weeks of age) and had no contact with an adult cat  may show this kind of behavior. This occurs because one of the many things feline moms do is to teach their kittens what scents, sounds, smells, and other stimuli they need to be pay attention to and what they can safely ignore. Lacking this knowledge, these cats tend to succumb to stimulus overload and over-react.  While some will just run, others will go into the fight mode and hiss as well as bite and scratch anyone who tries to interfere with them. In this case, it sounds like Slack may have ceded all territorial responsibility to Shoes who can’t handle it. The addition to two more cats would greatly increase the amount of stimulation Shoes receives and his stress level.

Providing him with a small carrier or kennel to which he can retreat  when he starts to get uptight, or scooping him up with a thick towel (to avoid getting injured) and putting him in such a confined space in a dark quiet room to calm down may help. A product called Feliway or its generic ComfortZone may also help.