silkies feathers dont hold heat as well as regular feathers because of their texture, they might need to be a little bigger (7-8wk?) hard to say without a photo.. also, you can offer heat in the coop at first to make the transition easier.
to introduce the bantam, it would be best if they...
Mine went out at 5 weeks this spring! They're doing great now at 7 weeks fully feathered. If yours are fully feathered, I think it's safe to put them out. They will pile up if they're cold, so just check on them the first few nights since piling can kill chicks. If you can teach them to roost...