Swanna and ducklet can only employ offensive and utility roles, they're not made for defense, so there is no good using defensive item on them. The best you can do on them, if you want something more defensive for that particularlly pokemon, is combine offense, defense and utility to employ ofensive roles with moves like Rost, double team, brave-bird, scald, etc. In that moveset for example, swanna has defensive move like rost and double team, but it still empoly ofensive role, because it combine double team and rost to regain health after a sucessfull bravebird, being hard to miss with double team. But swanna and ducklet just can't employ defensive roles, defensive moves on them are just for suport, not even their overal stats shows that, is like trying to use blaziken and Charizard for defense, they're just not made for this, so using eviolite on their pre-evolutions don't do any good.