Hey Sootyca -
I would not worry about the hissing overly much. I don't know that telling him "no" will help too much, but I don't think it can hurt. You mention that he hisses when you walk past him, and I am wondering if the hissing generally happens when you are "taller" than him, i.e. upright on two legs and moving around, and if he is generally friendly when you are sitting or lying down. That would be a very, very common behavior trait for a cat that was born to a feral mother. It generally improves with time, but can take several months.
As for 2 cats vs. 1. Absolutely get another cat, absolutely. They will probably share the box. The only thing I would want to point out is that the kitty that was born to a feral mom will probalby get attached to the other kitty faster than it will attach to you, but this too, will shift with time. Just try to make sure the second kitten or cat is very friendly with humans - maybe a bottle baby. Cats "pattern" after eachother and the scared/insecure ones will usually pattern after the secure friendly ones - at least that's what I see, and I have tamed about 3 feral litters this year, the oldest ones being about 4 months old. (No human contact prior to me handling them - and I have the scratches to prove it.) Actually, one of the litter didn't tame out, but that's a seperate post.
Good luck with the kitties! You are a gem for working with this little fluffer nut, I wish there were more understanding folks like you.
Esther