cat fights

Please help. I was calmly trying to introduce a stray to my 2 indoor cats. I followed all the advice about doing it slowly, etc. I was increasing time spent together which was always supervised. My oldest cat (17yr) doesn't seem to have a problem but my younger cat (8yr) does. She has been hissing and spitting and growling however, they have been known to be in the same room with each other and sleeping and doing other submissive type gestures. The other day I let the new cat out and she immediately ran and jumped my younger cat and they got into a horrible, horrible fight. The fur was literally flying!! Is the damage done or should I continue to try to get them together. The outside has claws the others do not. HELP.

To cut down on annoying phrasing, I'm going to call the 17 yr old "Oldster", 8 yr old "Huffy" & the stray "Bela" (which means "stranger" in Transylvanian).
   
I hope Huffy & Bela are alright. Treat scratches & minor wounds with Neosporin or any triple antibiotic twice a day until they scab over &/or start to heal. It worked on my gerbil, it'll work on a cat.
   
Let's not write this off yet. They've spent some non-combat time in a room together, so there's hope. I'm going to ask questions & will continue my major train of thought when I have the answers.
   
=How long has Bela been with you? How old is she? What's her personality like when she's away from Huffy?
   
=What did you let Bela out of when she jumped Huffy?
   
=What happened after the fight? I assume you broke it up? Who stayed mad longest?
   
I think you said that Huffy is hissing & spitting, & that Bela is a female. This is why hostilities are so intense - females are far more likely to deeply disagree over turf & behavior. I assume Bela is young, so the 2 young cats are clashing. Oldster is apparently above it all & doesn't particularly care (especially if Oldster is male, they don't get involved in girl fights).
   
Huffy is likely extremely annoyed that Bela's now in the house challenging her superiority. If Huffy starts the arguments, it's natural for Bela to fight back. Some cats will also attack pre-emptively, to try to get the upper hand - Bela's likely reasoning. Then, Bela keeps getting put in a cage, & she's blaming Huffy (who's probably throwing insults at Bela in the box). So, obviously (to Bela), Huffy deserves a whomping.
   
Right off, several things that may help:
   
1) Buy Soft Paws, flexible claw-covers for cats. I assume Bela no longer goes outdoors (& has had all her shots? Please tell me she doesn't go out... And that your other cats have been vaccinated for the bugs & parasites that Bela could bring in...consult your vet on this); this is essential for the Soft Paws to work. They're glued on the cat's claws & replaced as needed; they also come in several colors. This will cut down on the damage done during battle. Bela can't go outside w/Soft Paws, as they prevent her from defending herself.
   
2) Get an extra litter box & put it in a different place. Bela can use that one, so she won't be invading Huffy's personal potty space. Litterbox arguments are common.
   
3) Make sure Bela has a place to sleep that Huffy doesn't use.
   
One of the problems you're having is that the girls have to go through a progression of stages of feline politics. Some of it can be violent. It may be a good idea to go back to the beginning & start all over again with introducing them, but try something different this time. Putting Bela in a crate may be delaying the final peace talks.
   
Instead of putting Bela in a crate, when you're not around, confine her to another room behind a door with her litterbox & water. She & Huffy will sniff each other under the door when you're not around. It'll cut down on Bela's resentments.
   
Before I launch into an alternate "Get Acquainted" program, post again with answers & further thoughts.