I wouldn't say that the tank needs a snail. If there is that much uneaten food then the krib needs to be feed less. Most "cleaner" fish in freshwater aren't, they just can't get to the fresh food as quickly as other fish so they often end up living of what doesn't get ate.
Overall I would say it would be pretty iffy if a snail could survive with a single krib. I would say absolutely not with a breeding pair. But with a singleton, it will just depend on the personality of the krib. I've seen some that are very aggressive even when not breeding or another krib around, and others that are rather docile.