i know that no oranges to male rats, but you might as well not feed to either. no avocado that's from close to the pit, lettuce is okay but has almost no nutritional value so there's really no point unless you're just trying to get your rats hydrated. i heard that dried corn is bad, but fresh is okay.
you can google rat foods and you'll come up with a few pages with really substantial lists.
dried, it can also hold bacteria or something, so it can get your ratties sick. i fed it to my hamsters for 10 years and never had a problem, but i don't know if it's cuz they were hamsters or if i got lucky with corn, whatevah.
When we gave the ratties seed mix for a while (before coming here...) they rarely touched the corn anyway unless they'd gotten everything else. Fresh corn they love, though. Must be because it's sweet.