Sara if you have standard size two week and six week old chickens then you would be best off keeping your silkies separate. In fact since the Silkies are some of the smallest and most docile chickens around, you might want a separate coop and run when they get older also.
edit - you might NEED a separate coop and run because your bigger chickens will beat up your smaller chickens. Especially when it is Winter and they are all COOPED up together. It can be dangerous for a smaller chicken to be with larger ones. Not always, I know there are exceptions, but in general. Good luck.