the way i do it i have one female per bin. and i rotate the male from bin to bin on a weekly basis. i dont move the females out of their birthing racks, they stay in the same place at all times. when they give birth, i give four weeks until i wean off the babies. but i have a rack for birthing and a rack for grow ups, both 6 tub racks. i dont like over crowding but alot of people do a 1.3 colony per bin.
you shouldnt have a problem switching over to norway's. as long as they have the 1/2 hardware wire to eat through like you said you have they should be fine.
hope this helps!
1/2 inch mesh is to big...the weaners will be able to squeeze through it...1/4inch is big enough for them to eat, and small enough to keep all in the tub.