Will a silkie rooster fertalize an isa brown hens eggs?

Yes, cross fertilization between breeds happens all the time. They are all still chickens and if the rooster can manage to properly mount the hen then breeding will take place. Small banty roosters often have an easy time of mounting the larger hens and the mating will produce offspring. It sometimes does not work the other way around. When the rooster is too big for the hen she will not allow the male to mount.