I understand where you are coming from, however I do strongly believe it's good to be safe and supplement a feed intended for another animal.
If I were to choose a new brand of food to feed to my rats, I make sure to grab a complete listing of ingredients and their percentages from Harlan since I thoroughly trust their feed to be the absolute best in terms of a complete lab block, and try to follow everything as closely as possible.
I'm not condemning it at all, but it is something I feel strongly about. And not just feeding a rat some cheap old dog or hog food because Jo Blow does it and his rats are "doing just fine", but actually feeding them what they need to not only survive, but thrive under our care as we use them to produce our feeders.