Most of my customers used rice with EFM's.
As far as what type I don't think that should be an issue as long as you're not trying to burn rocks.
I've heard low ash makes big lumps, red ash burns too hot and plugs the grate
It's not the low ash that makes clinkers just some low ash/ red ash makes big clinkers, can't help you there as I'm not familiar with the different sources however if you get a low ash/white ash this won't be an issue. Coal berner would probably have a suggestion on where to get "clinkerless" red ash as that is hi neck of the woods if that's where you are getting it.