Langshans tend to be docile, sweet birds so hopefully they'll accept the silkies just fine.
You'll need to keep them separate but where they can see each other and if you can free range everyone together when you first introduce them into the same space as then there's lots of room for the silkies to get away if necessary.
Make sure the silkies crests are trimmed so they can see well and you have multiple feeding stations so the older birds can't guard one and prevent the silkies from eating.
If your coop is big enough you could net off an area for the silkies or use a dog crate or indoor rabbit cage. I'd leave them in there for at least a week before trying to let everyone mingle.
As an added level of safety you could put the silkies back into the cage in the coop at night and feed them there so you know they are getting access to food, and so they can't be bullied in the morning before you let everyone out for the day (if you shut the coop up at night).
I guess I’ll see how it goes 
