You can keep the cockerels with the pullets until they reach sexual maturity. As mentioned earlier, when you separate them, make sure they have plenty of room to get away from each other. Out of sight of the girls would also be good.
What will they need ? just a roosting pole?
A roost and different feed. Pullets need the calcium in layer feed, roos don't.
no nests exactly- do they just lay wherever?
They will sleep on the roosts at night and may hang out on them during the day. They will occasional lay on the floor during the day so provide some pine shavings on the inside floor for them.
Is a roosting pole better at a window or in the dark?
It basically doesn't matter unless you are in a hot climate and the sun shines in a lot. In a cooler climate, the birds will probably appreciate some sunshine when they stay inside on a cold day.
Use a large dowel? or strictly 2 x 4
Personal preference. I always use 2x4s with the flat side up. The birds can hunch down over their toes and keep them warm in cooler weather. Chickens also do not grip the roost the way most birds do. However, a lot of people use dowels and they work fine.
Silkies have a difficult time flying, so keep the roosts a little lower than normal. (There are exceptions.)