All Slivers have "When this permanent enters the battlefield, destroy target artifact or enchantment."
9/25/2006 The controller of the Sliver that enters the battlefield, not the controller of Harmonic Sliver, chooses the target.
9/25/2006 This ability will trigger when Harmonic Sliver enters the battlefield.
7/1/2013 Abilities that Slivers grant, as well as power/toughness boosts, are cumulative. However, for some abilities, like flying, having more than one instance of the ability doesn’t provide any additional benefit.
7/1/2013 If the creature type of a Sliver changes so it’s no longer a Sliver, it will no longer be affected by its own ability. Its ability will continue to affect other Sliver creatures.