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 Slivers owned by one player would die at the same time, that player chooses any of those Slivers and puts them on top of their library in any order. They don't reveal this order.

The replacement effect that Pulmonic Sliver grants to all Slivers applies to Pulmonic Sliver and any Slivers that would die at the same time as it.