For whether silkies can use the same nestboxes: they can probably use the same nestboxes if they can get to them (silkies don't fly, so they cannot reach high nestboxes).
Silkies are known for going broody, so you might need more than the usual number of nestboxes. When hens are not broody, three or four can typically share a nestbox--they just take turns. But a broody hen stays there all day long, which means no others can use that nestbox.
I have no personal experience with silkies, so I don't know whether they would prefer to sleep on the floor, in a hutch, or on a very low roost bar.