I've found that my ISA browns are very aggressive towards new members of the flock (yours might be different). Last year I added 2 new hens that I hatched, I made sure that they could see each other and that they were roughly of the same size. There was also a cockerel that I added and they had no problems with him. But the two new girls were constantly getting chased at the beginning. They're okay now but the ISA browns are still sometimes aggressive towards them and occasionally chase them around to let them know who's boss. I think adding 2 silkies to a flock of ISA browns would be difficult at first but it is possible, because silkies are know to be calm and docile I'd expect them to stay at the bottom of the pecking order. Try putting them in a smaller cage within the run and let them see each other but not touch. Then you could try letting the silkies out and see how they get along they might be fine or they might fight. I would however wait until the silkies are older and are almost fully grown. Good Luck!!