If the testicles are descended, you can have the bunny neutered. It is not necessary to wait until 6 months. It is just that 6 months is the average age that a bunny is mature enough for the testicles to descend. For small rabbits like netherland dwarfs 4 months is often more than old enough, where as with large breeds like flemish giants, you often have to wait till closer to a year before neutering.
You do have to keep the boys isolated for girls for about a month after their neuters, because they can still get a female pregnant. It takes a while for all the sperm in their system to die off after the neuter.
I would check out your local vets in case of a delivery emergency. Because the does are older, there is a larger possibility that they could get stuck kits or have other birthing complications. I would have a good vet on standby just in case something goes wrong. Nothing worse than trying to find a good vet at 1 in the morning when your girl has a stuck kit or is bleeding severely.
I think a divider down the NIC cage will work well. However, if you end up with babies, you will need to devise some sort of baby bumper as the holes of the NIC grids are big enough for little babies to squeeze through. I have double up the grids or used cardboard in the past.
--Dawn