I have raised cats for more than forty years. Mostly homeless who wander in, all ages. House rule is everyone gets along and they do. He might like a mom figure around or he may have a favorite buddy sibling, most do. Currently i have siblings and unrelated in my brood of seven. I am never without love or company. Kittens play together to develop self protection skills, it is nothing to worry about. Perhaps the container was too small. I don’t worry unless I see the fur flying and even that is usually a momentary dust up. Don’t be overly concerned over hissing at first, it is normal for them not to hit it off immediately, although that happens. And my mother cats growls at anyone, even her now five year old kittens, who get near her face. Think of what works for you. Mom will teach him socialization and hunting skills, manners, litter box etiquette. I think you are overly concerned. Just get them fixed and they should be fine. Good luck!
Most go for kittens. I usually save the older, less healthy guys. We all get along, siblings and newbees. One came from above Syracuse but older lady had two older males who rolled him and she did not want him living outside a second winter. Boy was he mad at being moved. Growled, bit my leg if I walked by him. I thought I’d finally met one I couldn’t handle. We had him and mom fixed at same time but once free they coupled immediately, I thought the surgery didn’t work. Well it all calmed down in a few days. One day he doesn’t come home. Days, weeks, months. I was a little hurt, I have NEVER had a cat choose someone else over me. My oldest male kitten was devastated. One day here he is at the back door. This time he “chose us” and no more growling, nipping, fussing. He is a real love bug. He went to visit his other family for about 24 hours the other day and the entire clan was looking for him, taking turns going outside to find him. He returned to a loving reunion and treats last night. Save the older female if you can. They will work things out. There are opinions but no exact formula that is absolute. They will take cues from you. If you are quiet and calm they will grow into it too. Just remember that little boy is a kitten, everything is new, fresh and exciting. He has lots of energy and curiosity now. Don’t holler or hit, chide him gently, pet him softly and tell him no, move him from the temptation. And enjoy his toddler years. They are not logical so don’t correct them like a child and expect it to work. Love them and let them love you. C*C