It's really not worth it to build a C&C for rats. Buy a Martin's cage or a Midwest Critter Nation. These cages are well worth the investment because they will last a very long time if cleaned and maintained properly.
A rat would be able to sqeeze throught the holes, however, if you cover them with hardware cloth you should be okay. Also keep in mind that rats chew more than guinea pigs and will start working on those zip-ties you might use to hold the C&C cage together. I myself have toyed with the idea of building a C&C Cage for my 7 rats but I think when I do build one, I will cover with hardware cloth and use as a playground, not as their primary cage.