I actually have that cage and it is currently housing my 2 "pet" boys. I'd say at most 3 adult males or females. It will work to house them when not breeding, but I'd agree, make a bin cage or use tubs for pregnant females... The pups I have now (got them when 4 weeks old) could easily escape the bar spacing in the rat manor.

It's an ok cage for pets though, but though to clean.