I would recommend a quarantine of a month if you have the facilities to do so. Even then, there are diseases that can result in an asymptomatic carrier state. It is a personal choice.
Silkies are vulnerable, but I have done lots of introductions and can tell you that chickens are individuals. So what you can do is after your quarantine is over and you are ready to integrate them, it is helpful to put them nearby but fenced off (like fence off part of the coop or run) so they can start the process of joining the flock. After a couple of weeks, try allowing the silkies access to the other section but stay there for a couple of hours and observe. 
If there is a peck or two and the aggression stops, that is normal. If the large fowl corner them and don't quit, or gang up on them and keep pecking at them with the silkies cowering, then it won't work at that time and separate them again.
I wouldn't worry too much unless you see the aggression at the time of integration (after two weeks of being close but separated).
Not everyone has facilities to do all this...so if you don't and you are going to put them right in be prepared to pull them out and it might not work, especially with your Leghorns and RIR. Some silkies I have had have actually been near the top of the pecking order, believe it or not. So they can hold their own but give them a fair chance so to speak.