I have five rats, all boys, and there are currently two cats (also boys, by some happenstance,
I would always keep them separate. If they are in their cage and the cat cannot reach into the cage or knock it down the rats will likely eventually get used to their audience and the smell of cat, but it will frighten them very much for the first couple days or more. If the cat can reach in... well, that depends on the rat... and the cat. Hennessy occasionally reaches a paw into the boys' cage, but uses no claws (despite having all his claws). Puck just avoids him, Jack's bit him once (a warning nip, nothing too bad.) Hennessy pretty much just watches them now, as does the dog. Lestadt seems afraid of them. Keep in mind these are boys rats and they're quite big (all over a pound, but for Bacardi, my smallest boy) and they were here first. The cat's in their home, not the other way round.
There are some cats with no prey drive that will ignore a free-ranging rat. There are some that will recognize a rat as family (or as a kitten) and ignore it.
But unless you know for certain that nothing can happen, I'd just keep them apart, you wouldn't want anything to happen to any of the babies... Remember... rat bites can be deep and vicious if the rat is being aggressive (if attacked) and could truly hurt the cat.