Hello! Are your older hens Silkies? It's always easier to mix all Silkies together than to mix Silkie chicks with standard breed chickens.
This said, generally you can mix chicks with adult birds at 3 or 4 months of age. Any younger and chicks can get hurt. Keep the chicks in a large cage or other separation pen within the flock at all times for at least 2 weeks, everybody sees but can't touch. After a couple few weeks mix them all together. Make sure to put out more food and water stations so the littles don't have to compete. Lots of levels snd hidy spaces. Any if an adult gets violent and hurts a chick, you may want to cage this bully for a couple weeks to let them all settle in. It can take a few months to get peace in the flock.
Good luck!