This is not recommended because the chicks will flog and peck each other- the larger chicks will usually bully and hurt the smaller ones. It is best to wait until they are full grown to mix the birds so they can defend themselves better.
Any time you mix a new group of chickens together they will fight to establish pecking order in the flock. To reduce this aggression, try putting them all in at the same time in a pen that none of them have been in previously. This will distract them from wanting to dominate their territory.