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.

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.

This card’s controller controls the triggered ability, but the controller of the Sliver that deals damage to a player decides whether or not to put the token onto the battlefield under their control.