Hi Kal
I had 4 newborn kittens which I reared as the mom couldnt, I kept two of them, Cole and Syra
About seven months later a friend gave my daughter a half persian kitten, Ruby. After around a week all three cats were friendly. Ruby was much more aggressive towards the older cats, who ran away rather than confront her.
A few weeks ago I rescued two kittens from the RSPCA, (step daughter works for them) Oscar and Morgan. Morgan is a tiny scrawny thing with long fur but he means buisness and was aggressive towards the other cats, Ruby didnt stand any nonsense from him and put him in his place, without any blood spill. Oscar is a very nervous little thing and just hides away, although he has taken to cole, very laid back.
Cole and Syra have both been neutered and speyed. The others will be when they are old enough.
I think its really down to the personality of the older cats. We put cat milk on the kittens and the others took to them within a week or so, Ruby was a little bit more aggressive towards them but she thinks she is too good for the likes us. I wasnt going to keep Morgan and Oscar but they all get on so well, we have decided to keep them all.
BK, the mother of Cole and Syra, was a stray, she hates all the other cats, we had her neutered but she doesnt stay with us, sometimes she will come for food but it can be weeks in between visits, she gets megga frontlined each time, so I cant say I blame her really.
Hope this helps.