Mbuna tend to ignore the juveniles, most are lousy but opportunistic predators. Under an inch, adults may be tempted to try to grab fry. Over an inch they may chase them out of the way but often ignore them totally. Juveniles are not ready to breed, are not a threat to the pecking order, and don't try to gain territory so there is little reason for conflict. Always thought adding 1.5" to 2" was the easiest.
Now adding fish at 2.5 inches can sometimes be a problem since they younger fish are now getting sexually mature and adults will see younger fish as competition, and males will try to breed with young females.