I have 6 Black Australorps, grown 3 white plymouth rocks, 6 game hens. The game hens raised their own so not quite sure about these new ones.
Let them get to know each other in the yard for a month or so and as soon as they are the same size or about 12-16 weeks old, you can put them together at night. There will be pecking, heads in corner, and bloodied combs, but this is natural.