I put three tiny day old Silkies in with standard chicks that were 10-14 days old and everyone was/is fine. The Silkies can walk under some of the older chicks without ducking but don't get picked on at all. From what I've read here, you just really need to watch for pecking and such. Because day olds can't walk well yet and the other ones were so much bigger I also worried about the new ones getting trampled. When I stuck my hand in to feed or clean the brooder, they'd all go running to the other edge and I did "rescue" the Silkies a couple of times from underneath, but I doubt it was actually necessary.
I think you just need to be prepared to perhaps have to separate them for a few days or longer if needed.
