Their 6 weeks old tomorrow and still tiny compared to the big hens.
How long should I wait, and when introducing them should I separate them for a while, but so they can still see each other but can't touch (slowly introduce them)
They need to be kept separately until 20 weeks, or until they begin to lay. Chicks need grower food, layer feed has too much calcium for their lil' systems. Hens need layer feed, grower food lacks the calcium they need to make good egg shells.
But, as they grow you can keep them in closer proximity to each other, to familiarize them with each other. They can even be let out to free-range together. You can put away the feeders to they can't get into each other's food while they're out. And they should all go to back to their own pens by themselves to roost.