As for raising silkies with other breeds, I suppose it can be done. Compare it to breeds of dogs. Think of the silkie as a dainty lil maltese. Your RIR's and leghorns are more like high energy labradors. They can be put together, but the circumstances are definitely against you. Silkies are social animals just like any other bird. If you keep them with the bigger ones, they are still going to want to hang out with them and interact. The big ones and even some of the feisty bantams, play a lil too rough for the dainty silkies though. They will most likely get hurt in the long run. Not even so much in the fighting aspect...just getting trampled. Silkies don't roost and are pretty much defenseless on the ground. Just about anything can walk right up to them. You also have to figure in the fact that most silkies now have vaulted skulls. That is basically their brain bulging up there and the hard plates of the skull don't always close. You can feel soft spots just like on a newborn human baby. Even so much as a hard peck to that can leave them with crookneck, brain damaged, or dead. I keep all my silkies with my pigeons only. The cages are for their own protection and they get left out under supervision only.
Amy