For 3, I wouldn't necessarily get a tank - I would get a high-defense DPS. If you get something like a death knight, maybe the difference is minimal. But something like a ret paladin would be fine (for that just pop righteous fury - voila, you hold the aggro off the squishy guys).

For 5, there isn't really a "best" and there are many things that work. Four shamans + Tank is an easy answer. Trinity groups in the form of tank + three dps + healer are usually fine too (with the usual proviso against multiple melee of course).