My Cochin rooster (black) weighed in at nearly 15 lbs. He's was a HUGE, silly, lovable ball of fluff. As for your Silkies, mine were that way too. They used to pick on all the other chicks in the brooder, but now they are sweet as pie. If you were planning on keeping them penned with that Cochin boy, its probably a good idea to get rid of them, however. If he tries to breed them (if they're girls), he could do them some serious harm bc of the size differences...
Nikki