Cat litter

I've read that rats can be trained to go in a specific area, and I know they sell little litter boxes to put in their cages. I also know that cat litter is not good for rats, but I've only seen that in reference to using it as a substitute for other bedding, so my question is, would it be okay to put in a rat's litter box? I ask because I have some cats, and it would be more convenient if I could just use a little cat litter for my rat's box ^_^