Cat food is too high in protein for rats. It can actually cause problems with weight gain, their kidneys, liver, and bladder. It does in dogs quite often. Cat food is one of those things that should only be fed to cats.
Dog food on the other hand is usually alright for ratties. It is recommended usually above all those fatty seed mixes you can buy in petstores. I use Nutro Lite for my ratties and make my own mix. Using low calorie dog food is ideal, but others like beneful and iams are alright as well. The lower in fat the better.
I feed my ratties Nutro Lite dog food, some seeds like sunflower, pumpkin, etc. They also get Total bran cereal flakes, dry oats, organic puffed rice (no sugar added), organic puffed wheat (no sugar added), organic puffed corn (very little and only occasionally as corn isn't the best for ratties), vegetables (fresh and frozen), some fruit, Kashi Go Lean cereal, and a few other things here or there.
They can also eat yogurt as a treat. And yogurt drops. On rare occasion they can have a little chocolate. They really like soy milk. I will even buy baby food and baby cereal and mix it with soy milk. And soy milk has seemed to make their coat so much shinier and smooth.
With dog food, low fat and low protein are key in feeding it to your rats. Just remember that with dog kibble your rats won't be getting enough Vitamin K (which is in the good quality lab blocks) so you will need to supplement their diet with foods that will give that to them.
I'm feeding my baby on Nature's Diet puppy (chicken and rice) at the moment, as well as him normal dry mix. My older rat has some as a treat but otherwise i find that a dry mix made of similar things to Rodere's in it is more than enough.
Puppy mix may be a little high in fat and protein since it's for growing pups. I've found that senior dog food is often the right amount of fat and protein... Inova senior has like 18% protein and 8% fat... but I might add that they're changing the formula to a higher fat, higher protein formula, so that kind won't be around much longer. I've heard a lot of people like the Nutro Lite
Well...rats are like living garbage disposals. Admitedly, some foods can be the rattie version of "junk food", but there's very few foods they can't eat. There's a list--but it's not very long. I feed Yuki & Choco scraps from the dinner table, ESPECIALLY chicken bones.