Time. It may just take a lot of time.

I have 14 cats and most of them were strays. Currently they range from 18 years to 21 months.

Older cats are usually fairly tolerant of the young but you will get play fighting and ranking fighting. The play fighting is just part of a kitten learning the ropes (and learning to respect the older cat.) The ranking fighting is where they work out their respective spots within their (limited) society and this can go on for a couple of years as the kitten discovers its own personality. Any changes to one of the cat's health can also result in ranking fights. Sometimes you have to intervene - you don't want them to injure each other (and certainly don't want a vet's bill to repair damage!) but otherwise just let them sort it out themselves.

If Bailey is male, Ebony may never like him but should learn to tolerate him provided they are allowed to keep their distance. For instance, my cat Red is only tolerated by most of the others and that includes most of his kids! He gets on the bed or couch, and the others give him a wide berth but still get up as well. Whereas others will lie together like a nest of vipers - all entangled up in each other.

Give them both their own spaces like feeding them away from one another. Make some time for each of them as individuals as well as joint play time. Don't force them to be together; let them work it out. You should try also to feed the Ebony first so she doesn't feel threatened by Bailey and you aren't intending to replace her with him/her.

My youngest, Pepper, was a stray - somebody's unwanted Xmas present, no doubt - and I have had her since she was about 3 months old. I just put her in the house with the others and let her settle in at her own rate, which she has done. She pretty much gets on well with all of them but gets put upon by the next youngest, Woodley (2 1/2 years old), as he starts to assert himself. He just barely gets on with his father, Red, and yet Pepper is adored by Red. They work it all out and I seldom have to intervene other than a shout. You may also find that Bailey gives Ebony a shot of youth serum! My 18 year old, Ginnee, goes running around the house with Pepper at times, completely belying his age and that he struggles to get onto the couch!

Biggest real suggestion I can offer is to make sure Ebony has somewhere she can escape from Bailey from.