housing...
I believe the rats would be more apt to bite the guinea pig, just because of their curiosity, which would make housing them together not a great idea. Also the two species need different nutrients, and it would be hard to keep an eye on how they are eating if they live together. I am not sure if there are any diseases that could be transferred, but anything is possible.
I myself have three rescued rats, but they never mingle with the guinea pigs.