Breeding rats on a small scale is pretty straightforward. You should do fine with 1:4 or 1:5 as long as you have an adequately sized cage. Keep in mind that if you don't separate the mothers before the babies are born, you may run into issues if you have several litters of different sizes. The older and larger rats will hog the mother's milk and the smaller babies may die. Also, depending on what size feeders you use, you may need grow out tubs for the rats to get up to feeding size.
I built some DIY Rodent Breeding Cages using plastic tubs and wire cloth. I'll post some pictures when I get a chance. Colder than 65 degrees or hotter than 85 degrees and your rats will stop producing babies. Rats probably smell a little less bad than mice, but you'll need to clean at least once a week and provide ventilation.
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