Doomsayer: At the start of your turn, destroy ALL minions. Dormant minions are still minions as they take up a minion spot.

Does silence work on dormant Hearthstone?

You can't target Sherazin when it is dormant. It ignores everything, including Brawl, until its owner wakes it back up by playing four cards in one turn.

Do dormant minions count as minions?

Minions will always start Dormant until their condition is fulfilled, no matter how they were summoned. The exception is when the player copies a Dormant minion that has already awakened.