With mice you almost have to have a harem, because continued pregnancies with no breaks between litters exhausts the female and she'll die. I'm not sure if this is also the case with rats, but it would make sense to me that you'd get better and bigger babies if the female had a month of rest between litters.
The best set up for small scale rat production would probably be a couple of females and one male each with their own cage (just drop the male into the appropriate female's cage for a week when you want her to get preggers), and a big cage/tank for raising up weaned babies if you want older feeders.
Rodents on the whole tolerate cool temperatures better than warm ones, so make sure they stay in a room that'll stay in the seventies and lower, rather than the 80's.
Rats REEK if you don't clean them regularly, so make sure you've got the time/energy to clean several rat cages every week or so.
Hope that helped!
~jenny
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1.1.1 normal ball pythons (Cindy, Darwin, and Periscope)
1.0 rex rat (Scurvy)
0.1 bunny (Spazz)
2.1 betta fishes (Vicious, Killer, and Butters)
2.2 great danes (Shasta, Odysseus, Merlot, and Watson)
1.0 fat fuzzy mutt (Smokey)
1.1 cats (Thidwick and Turtle)
2.0 horses (Buddy and Sam)
1.0 goat (Billy Jack)
2.25 chickens (Jacques the rooster and his harem)
but what I really want is more ball pythons!