We don't really know. For all the talk about the Twelve there's very little evidence of things like clergy or formal worship for most of them (outside of Ishagrd, anyway.) We see people talk about them and invoke them in oaths or the heat of the moment (by the Twelve!), and there are shines dedicated to them, and an occasional priest, but that's the extent of it shown in game. Outside of Ishagrd, prayer and worship seem to be more personal than organized.

We know that Au'Ra don't really worship any of them is it safe to assume Viera and Hrothgar will be the same?
Probably. Gods are very much a regional thing. The far east doesn't revere the Twelve just like Eorzea doesn't revere the kami. Stands to reason the Viera and Hrothgar worship something else entirely.