It depends on the individuals involved. Most of the time it works out fine as long as the chicks are within a couple weeks in age, but you don't want to attempt any additions after age four weeks.
When adding day-olds it helps to place them with the older chicks with a see-through barrier to protect them until they adjust and begin to know and learn the behavior of the older chicks. Chicks right out of an incubator don't have the experience yet to cope, but by the end of their first week, they will probably be ready to try mingling with the older ones.
It's not always the case that the aggression will come from the older chicks. A poster recently had a thread on here where the two-day old was the tyrant bully, jumping on and attacking the older chicks.
I have an article on how to cope with bully chicks linked below under "Articles".