Lets go the promotion rout. Also good point about the rebuking.Yup! We've got room to add more powers to it too. Anyhow, I think that's enough of an outline for me to start up a Working Draft.
I'd rather give them fire resistance/immunity and just the Spirit subtype.
The Cold Resistance and liability to being rebuked/commanded/bolstered by Fire Domain clerics of the Fire Subtype just seems wrong to me. It also seems wrong for fire clerics to be able to command Red Dragons with their turn/rebuke power too, but that's another matter.
That reminds me, should the Foxdragon have the Spirit subtype added to it or just the Fox Dragon version? I'm inclined to reserve the subtype to the true dragon version and say the Foxdragon's been "promoted" to that subtype.