I don’t know rats, but I have a bit of experience just from the times that my bunnies and I have, um…. had lots of house mice in our apartments, some of which I’d find in the bunnies’ area trying to eat their food. The bunnies didn’t seem to notice/care at all if there were mice near them, and the mice likewise seemed to know that the bunnies were not predators, and everyone just kind of kept to themselves. (Of course my buns have not been very territorial.) The main concern for me was that I didn’t want the mice getting germs on the bunnies’ food. Probably not a concern for your pet ratties, but maybe the food eating part might be?
Another thought is that bunnies are crepuscular (more active around dawn and dusk, generally pretty chill during the day), so you could probably have rattie playtime while the buns are in chill mode. I know some bunnies are more assertive/active than mine, but mine tend to hang out in their hidey houses for much of the middle of the day, and my guess is that if ratties were playing elsewhere in the room they wouldn’t bother coming out to investigate.
I guess I’m saying, I would guess it wouldn’t be a problem. But that’s just a guess.