glumshoe:

If you have rats, don’t let them interact with other small pets (or larger pets, for that matter). They can and possibly will attack, kill, and attempt to eat other animals of similar or smaller size even when well-fed, because they are instinctively predatory. 

Also, don’t let your cats near your small pets. Even cats who “would NEVER hurt them”. Cats have bacteria in their saliva that can easily cause lethal infections from something as small as a scratch from a tooth, a lick to thin skin, or the slightest nick from a recently groomed claw.