Well you have your tank and 1dps. The warlock curse should really let shaman shine.

For max syngery I would go with 2 more warlocks and a priest/pally for a healer

For max buffs I would go with 1 mage (int buff) 1 priest (stam/spirit buff) 1 ele shaman.

It is kind of hard to tell you what to do. I would think adding more classes would make everything harder but it would also make you that much more powerfull. Maybe look at theBigBB's setup he was doing fairly well with mixed classes in BC.