As for the maternity wards, the preferred is that rats pop the same day or 2 days apart. I only keep the incompatible rats 2 weeks apart, as in-not cagemates.
I keep about 14 females and I have 25 extremely well fed snakes (2-4 baby rats per snake a week.), a well stocked freezer, and well rested, healthy rats.
As I said before, the 10 gallon aquariums I use for maternity wards are ok for a pair of females because they are concentrating on their young, it wouldn't work for 3 females. I don't have any wheels for the rats, and their muscle tone is fine. Rearing babies is strenuous work. They won't use the wheels in the maternity wards anyway, too busy taking care of their babies.
I use 55 gallon aquariums for the breeder tanks, and only have 1 2 or 1 4 in them. I don't see any reason that a 40 long tank wouldn't work, but I got good deals on used 55 gallon tanks from the local paper's bargain corner. I paid $100 for 2 55 gallon tanks and stands. The extra height is good for bigger water bottles, and minimizes cage climbing.

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