I also used Clays design and currently have 5 racks running with 6 concrete mixing tubs each. They are each stocked with 1.5 breeder rats.
I do not pull pregos, and have been doing it this way for about two years.
I leave the same 1.5 group in the same tub for life, and do not have many issues.

What size are your female breeders?
I have found that female rats under 250g do not make good mothers. They will eat their own litter.

If you touch the pinks with your bare hands, some males, or even some moms will eat them. I buy the cheap plastic gloves at the dollar store for moving pinks during cleaning. (They are $1. for 100ct.)

I have 1 rack with 6 concrete mixing tubs stocked with 1.5 ASF's running the same way, and have no issues with them either.