So that is my first problem. do i go with hunter and mage? or mage and lock? or hunter and lock?

You already have 3 shaman (make 1 resto) so you need 1 DPS heavy and 1 Tank. I chose lock simply because of the following:

Pro's

(1) Online DPS meter 25 man Ulduar
http://www.wowmeteronline.com/rank/c.../dps/7/0/3/310

I made total DPS and average of 6 long range DPS class and lock leads followed by mage

(2) Lock summon stone - with this you only need 3 flying mounts and just summon the other 2 anywhere you want.

(3) Crowd Control

(4) Pet, DOTs and AOE

(5) Great for PVP


CON's

(1) Cloth (where Hunter, Druid and Shaman have the advantage)

(2) No food and water (Though you have 3 shaman for hp/mana regeneration)



Now If you wanted to make your shamans DPS then get a druid for healing and another druid for tanking. When your guild needs 5man in raids, your 2 druids can shift to boomkin for a 5 man long range DPS team.



DK tanking
I have both DK and Paladin and you can check a lot of thread with regards to this matter. The advantage of having a DK tank is you deal more damage and your DPS team will deal more damage from Ebon Plaguebringer, Pet and Army of the dead and good enough magical defense. Problem is since Day 1 of Wotlk, Blizzard has been strucking this class with the nerf stick particularly in tanking area because a lot of people are complaining how OP it was not considering that this is a Hero Class so it should be that powerful. But if I where to choose I will stick to paladin. Paladin has shield and gives a lot of buffs and support. It has some OHSHIT buttons and really good in handling threat.