As others have said, play what you like, the server population isn't going to affect you all that much if you have someone to play with most of the time.
That being said from an exp perspective on a cleric/shaman duo you are going to be hurting for dps probably well into the 20's. I find myself sitting on my arse a lot just waiting on mana because my offensive skills are garbage and my dots aren't that great. Cleric isn't going to help much in that department either, you could do undead camps to make this better but you're still relying on mana for dps on both classes. You probably have a fairly safe group with all the healing power available so there is that.
If you are thinking of having a good base for a group with the option to duo however, cleric/shaman duo is certainly a good group base and you can just duo until you find others for dps/tanking...
Anyhow as you can see plenty of options just depends on how you plan on playing, the main thing is to just pick what you will enjoy then the rest of it won't matter so much
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