How long did you wait before introducing them? The best way to introduce cats is to keep the newcomer(s) in a seperate room and allow the resident cats to get used to the smell without physical confrontation. This process should take about 2 weeks (hehe kind of like cycling your cats). If you didn't do this then you might consider doing a "reintroduction." You can often rub the cat with a towel and put the towel on the outside of the door (full of new cats scent) for the other cat to check out.
Then get some Feliway diffusers. Many people sware by them. You can put feliway spray around the door or get a diffuser for the main area your cats reside (or during the introduction in the bedroom of new cat).
Encourage the cats to play footsies under the door. You can do this by using toys. Or if they are not toy motivated then use treats when they are near each other.
Once you are ready to introduce them (in your case waiting a month might be more appropriate) then associate positive things with the introduction. Put out their favorite food. Give them lots of attention and play with them together. You can even do "play sessions" everyday, only let them be together during play or feeding. Again positive reinforcement. Presence of offending cat will eventually be a good thing.
Right now you are seeing the territorial fighting most likely because top cats territory was suddently invaded like others have said. New cat is scared and acting defensive, spurring them into a cycle that probably won't end without help. They really need a warm-up period.
They might never be grooming buddies but they certainly should learn to tolerate each other.
We have 3 females (usually a harder combo than males) and our oldest cat (2) still is not fond of our kittens. However, she is tolerant she will just give them a swat or hiss if they get to close for to long. This sort of thing is just reinforcing to the kittens that she is still top cat and nothing to worry about. So if you get to that point then that might be the best you are going to get.
If a real cat fight breaks out throwing something over the cats will usually stop it. Or it will stop it enough for you to pick up offending cat inside of coat/towel/etc. When a cat is blinded they usually will submit to you and makes seperation pretty easy.
I know a lot more about cats than fish as they are my #1 hobby.
Here's a link to feliway. You can get it at Petsmart/Petco but it is pricey. It usually cheaper at Dr. Foster & Smith or online stores.
Feliway Spray
Feliway Diffuser