Did your rooster grow up with the Silkies? If he did, that may be why he's so okay with them. If not, then he must really gentle and laid back and adding new banties might be okay.
On the other hand, it might be a really bad idea. I say this because I've had experience with it. I tried to integrate my bantams in with my full-size flock because winter was coming on and I decided it was time the bantams didn't just wander our property all day. I had tried integrating them earlier in the summer, but they just escaped as soon as I put them in, so I just let them be "yard birds". Anyway, when I put them in with the big birds a few weeks ago, I hung around to supervise and make sure everything went okay.
I expected some aggression as pecking order was reestablished and I provided hiding places for the three bantams, but what I didn't expect was to see my four cockerels (going to be used for breeding so have to keep all 4) try to KILL my bantams. They had one of the little Sebright girls like she was a mouse they found and they were ripping her feathers out and playing tug-of-war with her while she SCREAMED. Well, let me tell you, I jumped in and rescued her and took the other two out and they are now back in their old house--my husband is reconnecting the electric to it for the water warmer. Poor little things. =(
