Incidentally, Elves (Elfi) are well known in Italy, whereas Nymphs are practically unknown in popular superstition. People must understand that Italy is not ancient Rome, much less ancient Greece! A great deal of Italian folklore is of Teutonic descent, although that should not be taken as a rule. Mainly, because of the whirlpool of different traditions that Italian culture history has been, you end up with quite original and very local matters. But a "house-elf" - elfo domestico - is perfectly appropriate, especially since domestico is a frequent expression for a house servant.