"Rat" and "informer" IS the same thing. A person can be a rat and not necessarily end up being a government witness, although it usually leads to that. You have to be an informer before you can be a witness after the RICO charges go down...

I wouldn't call Larry a rat. He may have thought about turning on Tony and the crew but he didn't. He gave false information to score himself points and still protect his pals at the same time.