If you're questing, your composition won't matter at all.
1x + 1x + 1x + 2x is as good as 5x.
It all comes down to your DPS macro, that doesn't have to be that complicated.

5x Shaman would rarely kill anything with 5x Lightning Bolt, but 10x (a second cast on each) would kill anything.
Conversely 5x Penance or 5x Smite, would drop almost anything, but the second press of the DPS key guaranteed a kill.
If the macro was Lightning Bolt + Shadow Bolt + Fireball + Penance + Wrath... one or two presses will kill whatever.

The only consideration, is everyone having some kind of a DPS spec, and contributing to the kill speed.



You'll likely need a Tank type for a lot of instances.
And then either three/four hybrid DPS who can heal.
Or three DPS who just DPS, along with a healer class/spec for healing.

Instancing is an option, and generally works better this way than with 5x DPS.
But a lot of 5x One-Class can do instances if you want.