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.

If multiple replacement effects apply to a creature being dealt damage, the controller of the creature chooses the order to apply them, not the controller of the source of damage.

If multiple sources would deal damage to a Sliver at once, prevent 1 damage from each of those sources.