If someone somehow swallows a spider or silver fish while sleeping, what harm can that pose to that person?

Probably nothing: Unless the spider is poisonous and bites the individual or the individual has an allergy to either the spider or silverfish, in all likelihood nothing harmful will happen. The insect/arachnid will die, to include being completely obliterated by stomach acid, and processed like any other substance entering the gut. In some tribes and cultures insects are eaten as a part of their regular diets.