First of all, thank you for rescuing this kitty.
Perhaps a stupid question - but he has been neutered, right?
Three years is a long time for him to have been outside, and it's wonderful he's adapted so well to being inside!
There ARE things you can do to help - but the number one thing is just going to be time.
The cat intro piece Yayi refers to is here: http://www.thecatsite.com/Behavior/4...cing-Cats.html
It's a good piece, but there's lots of other ideas.
1) Do you have an upstairs room in which black feral can be confined with screen door? May make proximity issues easier. ????????????
2) Feliway plug-ins or spray. Feliway mimics the "friendly" markers in cats' cheeks, and it helps promote a calmer environment. Do NOT spray near litter boxes or where they scratch.
3) Bach's Rescue Remedy (Flower Essences) for all the kitties. Sometimes it works, sometimes it doesn't - but when it works, it helps. If it works, it'll help destress existing cats, may help aggression in feral.
4) Rub down all kitties with clean rags (best if washed with no fabric softener). Put existing kitty scented rags next to each other under food dish of feral. Put feral scented rag under food dishes of existing kitties.
4) Swap cat beds between them - get feral scented stuff upstairs in places your kitties associate with comfort and happiness. Get existing kitty scented stuff in places feral is happy.
5) If they all react to catnip, get new catnip toys. Give one to each - let them drool all over them. Give a second one a few days later to feral. Then put them all away for a week, keeping track of which one went to who. Then give each of your kitties one of the catnip toys that was drooled on by your feral, and vice versa. (Don't leave catnip toys out - the cats become desensitized to it, so always put them away for a week or two after the day they were out).
Do all of this for at least a few weeks before you even begin "encounters" between them again.
You can try getting creative with this, but here's the basic jist of introductions. The idea is to get the cats associating each others' smells with things they love, with things that are happy and comfortable. With treats, with play, with brushes, with pets - whatever motivates them. If they all like being brushed, brush feral - then use that brush on your kitties. Etc. Scent is the best way to do this. Give it AT LEAST a few weeks before trying actual interaction again.
After that couple of weeks (or month or whatever), instead of introing them again, you may want to "swap places" for a few hours. Let feral upstairs, take kitties downstairs. Let them explore the other scents. Maybe do this each day for a week or so.
When attempting reintro with actual interaction, I'm not sure. I would consider a large crate or cage for the feral in whatever room the kitties are most comfortable. Maybe for 15 minutes or half an hour. Let them see/smell him (if they even get near) - but the idea is if you do this daily for a while, they have the ability to see him as a non-threat. If there's hissing, give it a break, try again a day or two later, and just keep up with the scent swapping. Get more rags, put treats down on feral scented kitty rags for your existing cats.
The idea is also to reinforce the positives - you want your existing kitties to think having the new cat around is a total party. They get extra treats when he or his scent is near. He gets extra treats when he doesn't hiss at them from his cage or at the screen door.
The idea is also to stop all interaction when he's being aggressive. Lunging at your kitties means he goes back to his room or back downstairs with no attention from you for a while. It's like a "time out" with a young child.
Now - all of that said - they ARE going to have to work out the "cat status" thing, and as Yayi pointed out, it sure looks like he's going to establish his place as alpha. Some fighting is normal - however, having been outside for three years and not "knowing" "polite" cat communication, let's call it, you are going to have to limit the interaction and give more time to this. But at some point, if no one is being actually hurt, you will have to let him establish his dominance (if that's what's happening).
Also - do you have vertical space? In dealing with new intros and cat hierarchies, a REAL help is LOTS of vertical space. Cat trees are great. Don't know what your set up is, but if we owned a house, we would so get creative with it. Check out this stuff: http://www.katwallks.com/
Also consider PMing Skippymjp - he's gotten really creative with rubbermaid containers. I can't figure the right search words to find pics of his great rubbermaid container condos, but I'm sure he'd be happy to share his ideas in that regard. But the more vertical space you have the better. If they're not used to going vertical, get your existing kitties playing on the new vertical spaces so they're totally comfortable running/jumping up. Also, with the ability for the alpha (whoever it's going to be) get up high also helps reduce physical stress of establishing that cat heirarchy.
We just adopted in November a semi-feral kitty that had lived outside for 1 1/2 years or a little more. He wanted to play with our cats - he did this by lunging at them. He was not attacking them. (Though if we were new to this it sure would have looked like he was attacking them). It took about 3 months for him to "learn" that he needs to be less aggressive about it. Our existing gang aren't marshmallows though, so they'd bonk him back, grapple with him and roll by running away. That didn't wind up with what he wanted - continued interaction. So he's learned.
But when you do get to the point you think interaction is worth a go, keep an empty can with coins in it handy. SHAKE IT if he lunges at someone - it startles them out of the "fight" or whatever it is. Do not EVER attempt to break up a fight (or play that looks like a fight) by picking up one of the cats - get them startled out of the grapple first or you may get hurt. Then give him his time out, and reassure your existing kitties.
Hope this helps! Love it if you keep us posted!