I also have the same group composition, but horde side.

Couple of things. Don't go Holy/Disc until you start doing heroics. You won't need to. I don't even think I healed as I leveled up. You probably already know that =)

Another thing I noticed is that my shaman was the easiest to gear and increase the spellpower. You can maximize their DPS without any micromanagement. I'd consider using two if I ever did it over again, esp with dual spec.

The mage was the hardest to raise spellpower. It is still behind my boomkin and shaman in DPS, but should be ahead. Maybe it's just me, I don't know, but the mage has been the hardest to gear up etc. After getting to the point I am, I can easily see how a Tank + 4 shamans would totally own. Part of the reason is that Shaman can use chain, leather, cloth and boomkins leather and cloth. Having the ports, food and INT buff probably makes up for it though.

I'd also stick with a human paladin. Being short sucks IMO (so does being too big....Draenai and Tauren, suck it up and go female).

The only alternatives IMO:

Tank - Paladin OR DK - both have been demonstrated to be great tanks, I'd pick the class you'd prefer to play solo with consideration of the off spec
DPS1 - Mage - ports, food, INT buff, CC
DPS2 - Shaman - totems!!
DPS3 - Boomkin OR Warlock OR Hunter OR Shaman - I think any of these would work fine. Boomkin and Shaman have great off-specs to change their roles (tank or healer) which makes them better choices IMO.
Healer - Priest - AOE Heals