They start low and work their way up. Be sure they have a roost close to the ground to start. Mine learned to roost on a couple of logs put in the chicken house. Within a week they were up on the six footer.
I agree it is a good idea to board up the nesting boxes. They won't need them for a couple of months and it is a bad habit that is hard to break if they use them as roosts.
Your silkies may never roost. All of mine prefer to sleep in a nest box, on the floor, or on a board i have on top of the nest boxes. But i had to put a ramp in the coop so they could get on top of the board. Silkies can hop and flutter a bit, but can't fly, so not very good at getting up to a tall roost. Your Wyandottes may be hanging down with the silkies to stay warm.