two of my cats are like oil and water

We have four cats, and two cannot seem to adjust to each other. All are from shelters at different times, so I do not know what their earliest experiences were. All are also spayed or neutered. We have an approximately 14 year old male, Meeko, who is mostly a loner but can be affectionate with people, only. Then we have a female, Sassy, who is approximately 3 years old,  a female, Foxy, approximately 2 years old, and a male, Friend, who is about 3 years old. Friend was a feral cat, and part of a colony of community cats I feed with a few other neighbors. A woman who has been feeding for 11 years says he was born outdoors and was never in a home, but he was so friendly and engaging that I trapped him and brought him home. He's a great cat and is loving and intelligent, but has become the Alpha male. Sassy's personality has changed since I brought Friend home. She used to be somewhat distant, but enjoyed being petted for short periods of time. Now she's skittish, avoidant, nervous, and goes after Friend constantly. I introduced them after about 10 days, and that may have been too soon, but I was on vacation at the time so could keep them separate and give everyone attention. I have too small a  place to lock one of them away all day while my husband and I are at work. After about a week of being together, Sassy started attacking Friend. At first it was just a few swats and a growl or hiss, but now it's just short of the fur-flying stage. The shrieks happen several times a day. We've tried a calming spray, feeding them separately, giving each affection (which Sassy mostly runs from), and introducing an extra cat perch Any ideas? Re-homing is not an option. I don't know anyone who I could trust to keep her, and besides, we really want to find a way to make this work.

That is a tough situation. You should try getting some Feliway diffusers. They have the same pheromones as most calming sprays but disperse the scent continuously and more evenly. Also make sure there's enough toys around to keep Sassy busy so she won't attack Friend out of boredom. You might also try a toy that dispenses kibble or treats to keep her entertained. My female cat Ingvild Solveig beats up my other female cat Sniffles and all of these are things my vet recommended. They helped my situation quite a bit along with prescribing fluoxetine (Prozac) for Ingvild Solveig. Hopefully your situation gets better as the two become more accustomed to each other. Sometimes it just takes time for cats to figure out their place on the social dominance ladder. Good luck!