Harmonic Sliver

• WWizards of the Coast2013-07-01
  Official Ruling
  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.
• WWizards of the Coast2013-07-01
  Official Ruling
  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.
• WWizards of the Coast2021-03-19
  Official Ruling
  The controller of the Sliver that enters the battlefield controls the triggered ability for that Sliver and chooses what to destroy, not necessarily the controller of Harmonic Sliver.
• WWizards of the Coast2021-03-19
  Official Ruling
  The ability Harmonic Sliver grants itself triggers when it enters the battlefield.

Rulings via Scryfall