I work in animal rescue, and this year I have been socialising a large number of kittens for homing. Due to space problems, I had to change their accommodation and cage-mates around several times. Sometimes I mixed three or four kittens unknown to each other: at other times I introduced a new one to an existing sibling group of two or three. I never had any problems. The kittens ranged in age from 5 weeks up to 18 weeks. As long as you don't have too big an age gap, which might encouage bullying, you shouldn't have any problems.
Just be careful however that the newcomer does not carry any viruses or parasites that he might pass on.
Best of luck.