I'm new to chickens, but I was forced to put my 2 (5 day old) Silkies in with my 3 (about 3 week old) MONSTROUS sized Brahmas and 3 (about 2 week old) EEs.
They were a little too curious when I put them directly in, so I separated them by a small screen for 2 days first, and got them used to eachother when I got them all out to play in the larger area of my kitchen with the Silkies on my lap, just so I could rebuke any roughness from the older chicks (oh yes, my chickies are very smart! they learned not to peck the hands within a few days of gentle taps on the beak whenever they pecked, and they stopped showing over-interest in the Silkies within a fwe sessions).
In a few days I put them all together, and never had ANY problems. Nobody ever lost a single feather. The only thing I had to watch for was the big birds got a little peckish if the Silkie's bottoms pasted up. As soon as I cleaned them, they were fine.
They were only together for a few weeks, at which point I was able to get more supplies and separate them (I was afraid of the Silkie's getting crushed by my clutzy, if sweet, Brahmas). The only thing I was worried about was the Brahmas leaping of their perch and landing on the Silkies, who were so tiny they could have been badly hurt.