I'd say if your BP and BCI are taking two adult mice, they could easily handle a young rat pup. And rats are nice in that they have more bang for the buck usually. More nutrition in the same size package.
The cornsnake on mouse fuzzies could take a rat pink, as they're just about the same size. Even with the hair and more solid bone, the mouse is a little less for nutrition than the rat pink.
I would think a 1:1 of rats could feed your snakes nicely. But I'd still worry about excess rats not being used. A rat can give birth to 10+ pups, so you factor in you have 3 snakes now..and maybe 2 more coming. So by the time some of them got grown to the size for BP or BCI size, they'd be close to popping more out. If you feed f/t, then just euthanize and freeze the babies as the size the snakes need or increases..and if the adults are producing too many, just get rid of them as well after you've stockpiled a supply in the freezer.
Of course, getting more snakes alleviates the waste issue. Must have more snakes to eat all of them mice.
As for rats in a plastic tub, I wouldn't recommend it. They can chew through just about anything, and if they can get an edge, it'll have a hole in a few hours they can fit through. Not to mention tubs such as that hold in moisture and odor.
I would recommend a wire cage or a large aquarium. Rats are prone to respiratory issues and if not cleaned out often the ammonia in their waste can be a health issue. The best luck I've had is keeping them in a wire cage. Its easy to clean and provides good ventilation. You can build your own from a rabbit hutch design or buy like a ferret/guinea pig cage. Although you might want to buy some 1/4" to 1/2" hardware cloth to cut in sections to fit over the wire of the cage. The babies can fit through most wire cages available at Petsmart. I got a Midwest rat cage and love it.
These are rats, not snakes. They produce copious amounts of pellets and urine, newspaper I wouldn't recommend as a substrate for a variety of reasons. It is absorbant to a certain degree, but would need changed every day. The urine gets absorbed sure, but it doesn't really go anywhere. What with the incessant waterbottle drip, urine, etc.
Aspen litter or a paper based litter (Carefresh, PaPurr) works great. Its absorbant, deodorizing, and diggable. Rats like to dig and make nests, you'd quickly find your paper shredded and in one corner while they poo and pee on the bare floor. And the ink could get on the rats. Snakes don't eat the ink on the paper in their vivs, but I don't think that ink is recommended for food items generally.
And I'd recommend a 1:1 to start out. For all of their vicious nature that rats have, you can always add new females later on if you need more food. They're not like mice and fight to the bitter end. They'll have their "hen-pecking" initially, but it'll pass in a few hours once the newcomer knows where she rests. Or you can always raise up a female from the litter previously which I use often, as it avoids conflict.
