Some things depend on what works best for you. For instance, when my raid was doing normal Halfus we needed Time Warden out ASAP because the tanks were doing fine but the raid was taking too much damage.
Generally speaking, Slate can be ignored. Pulling two drakes is usually best but not always necessary, depending on the drakes. For my group, again, it was easier to have the tanks take the damage from multiple drakes than to have the raid taking the damage.
If you're pugging 60% of the raid, bring three healers. Three healers will be best until you get more experience or higher gear levels.
The Halfus debuff can be dispelled by paladin abilities but it will wipe threat on that tank, so your paladin and tank will need to coordinate so the tank can retaunt right away.