question on the title
Yes, they do, typically you go with 1-3-6, so tanks are sometimes on demand if a group has been waiting for ages to fill the last spot.
usually DH tanks.
DH, maybe a Guardian Druid. Rest is bad, it's mostly for CTF as Flag Carrier, picking the wagon in Deepwing gorge and doing a solo cart in Silvershard mines. Usually go back to havoc / Balance if you're in another bg.
Usually DH. Can run blood dk too, but it's very rare. I've also seen a BM monk used effectively at higher mmr
Working on my next ban.