Yes 20"*27" is big enough for a mom and her babies but not for 5 females and 2 males and then their 5 litters on top of that. Better off going with a trio, or just a pair. I go with pairs in 18.25" x 12" cages.

You said you're gonna start with 5 females and 2 males and want a rack big enough for a mom and her babies. So you mean you're gonna pull a pregnant female and leave the other 4 females and 2 males? or are you going to pull her after she drops a litter? Either way I think is bad, I learned the hard with my African Soft Furs that a lone, isolated female will die. Sucks to wake up one morning and see a big, fat, plump, pregnant rat that I was eagerly gonna count pups... but she doesn't move coz she's dead. (May be different with common rats.) Also pulling a nursing female together with her litter, and one of her sisters, didn't work either. Dead pups next day. Nursing mothers best left alone. Again could be different with common rats but why push it?

The idea of interbreeding is to avoid inbreeding and keep the litter sizes large and fast growing. But IME some interbreeding is good. We don't want to mix a rat that, for example, comes from a line whose litters are consistently 20+ pups with a rat from a line of small litters. Also, if/when you come across a unique color don't mix it with common colors, instead try to create some more of that line via inbreeding. Rats of uncommon appearance, e.g. solid beige w/ruby eyes, can fetch $10+ each on craigslist in my area. And though I didn't get into it for the $ it does help to offset costs of food/bedding.

Consider setting them up outside and/or in an area where the stench (of common rats) won't bother humans.

Depending on how near a population center you reside craigslist can be helpful, and economical, in obtaining the supplies that you will need to get started. Seems like for every guy/gal that is getting into the hobby there is one that is getting out of it and looking to unload his/her equipment.

Good luck and have fun with your new endeavor. I like rat breeding more than snake breeding.