Shadow priests are fine especially with their support buffs. Their only downside, when it comes to heroics, is their lack of burst damage which comes even more apparent when you run with elemental shamans.

I've been running with 2 elemantal shamans and a spriest for quite some time now and due to their gear and how quick trash dies (heroics) my priest hardly ever casts any dots.

In the end it comes down to personal preferences and shadow priests are more then viable.

Regarding your healer I would rather go for a shaman one and spec your druid to balance which goes quite nicely with your shadow priests (low burst, great dots).