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.

Unlike a normal cycling ability, Slivercycling doesn't have you draw a card. Instead, it lets you search your library for a Sliver card.

Slivercycling is a form of cycling. Any ability that triggers on a card being cycled also triggers on Slivercycling a card. Any ability that stops a cycling ability from being activated also stops Slivercycling from being activated.