Simply not true, at least in our colony. We routinely keep 2-3 males in one cement tub, and have had 4 in one tub at times for a day or two at a time with no problems, regardless of if one of them recently bred.
As for pricing, if they are quality female rats, 5 bucks a piece is fine. The ASF's seem very expensive, I would talk them down or find a different supplier. IMO, an ASF should be no more than 5 dollars as an adult.
Also, I would not birth more than 2 in a rack our size (not sure how this compares to yours)--ours measure ~26x20x6. Just to breed, 4 females would be probably be fine.
Also, if you have not ordered your water bottles yet, I suggest you use a watering system instead. I recently made the switch and it was the best decision we have made, apart from building the rack in the first place. The noise factor is a lot of it for us, but not having to fill bottles all the time is definitely a huge perk.