Hi Annalou
When I introduced Marble to Bourneville it was quite horrible and worrying for me. It took around a month of keeping them separate and doing introductions before they started to tolerate each other and even when they got better they would still fight and I was worried Marble would get hurt cos she was only a little kitten. How about swapping over who is in the safe room, I did that. So for an hour have kitten around the rest of the house and nipper in the safe room on his own so he can do whatever he needs to do to make himself feel better about the newbie. Do that for about a week and that should help mix their smells thoroughly.
Have you got any feliway, zylkene or rescue remedy yet? These things do work and will help to calm the situation. I strongly recommend that you get one or more of these products.
After room swapping I would say just let them out together for short periods. Its not always pretty but you have to do it so they sort out their pecking order. There will be growls, hisses, spits and swipes but they have to go through it. It worried the life out of me with Bourneville and Marble but they were find and eventually Marble became leader of the pack.
When I got Soap earlier this year I only keep her separate for a few days and then let them sort it out between themselves which they did. Soap was an extremely confident kitten and more than capable of holding her own.
Soap is still trying to battle Marble for the "crown" so they will never be best friends and still have a daily spat but they tolerate each other. Unfortunately not all cats will be best friends but they will find their own spaces and will live alongside each other.
I hope that helps, just try not worry. They will get rough just keep an eye to make sure they are not murdering each other and they'll be fine