First of all let me tell you how beautiful Sirocco (I saw his pics in your other post) and Greta are.

I'm not sure if I can help you but at least I can tell you about my experience, in recent years it has happened quite a few times that I had to introduce, sometimes even temporarily, cats in an existing group. I won't bore you with all examples, I'll just talk about my recent experience.

When my 19 year old male died in September a short time later we adopted a 2 month old kitten. My 2 females reacted very differently: Prema would not have anything to do with him, would hiss, growl and try to hit him. In the beginning he tried to approach her but in the end he got so scared he would either hide or freeze on the spot. In this case Maya would come to his rescue and send Prema running. Maya sort of took over the mother role even though we was (and still is) a true pest with her, he just won't leave her in peace.

After a month though the situation changed completely. Foxy is no longer scared of Prema and she accepted him. He steals her food, jumps on her just as he does with Maya. During the night he often sleeps next to Maya or, now that he has obtained her permission he sleeps with Prema on my bed (now that he has obtained her permission . Prema and Foxy now love to play together, quite rough at times, but if it's not one that it's the other who instigates it. During the day Prema and Foxy are often found in the same room and they enjoy each others company.

Maya on the other hand tends to ignore them both and prefers to be on her own.

I don't think there is a rule, I personally would advise you to give them some supervised time together, let them get to know each other at their own pace, just separate them if things get a bit too rough. Good luck and keep us posted.