Abilities that Slivers grant, as well as power/toughness boosts, are cumulative. However, for some abilities, like indestructible and the ability granted by Belligerent Sliver, having more than one instance of the ability doesn’t provide any additional benefit.

If you change the creature type of a Sliver you control 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 you control.

Slivers in this set affect only Sliver creatures you control. They don’t grant bonuses to your opponents’ Slivers. This is the same way Slivers from the Magic 2014 core set worked, while Slivers in earlier sets granted bonuses to all Slivers.

Damage dealt to a Sliver with indestructible is still marked on that creature. If it has lethal damage marked on it and it loses indestructible (perhaps because Sliver Hivelord leaves the battlefield), it will be destroyed.