I have a mixed flock with several bantams and about 18 standards with my big RIR rooster, he is fine with them all, the bantams stay away from him and he doesn't have much interest in the banties either so it works out great (the rooster can hurt the bantams as he is much bigger than them).
Langshans are a very economical eater, a great forager, they are big enough to not be easily attacked by hawks, are great flyers so can keep them selves safe, are very friendly and are great hardy layers that will lay through winter. Sounds like the perfect breed for you.
Side note: If you have drakes they do not mix well with hens as they can badly hurt them when they try to mount them.