Those new Dragon templates look good Buddha. Have you looked at the master Dragon List someone is composing in the General Forum? Of course, that's only if you're trying to be a completest and collect all the half dragons types.
The Hellfire Wyrm is pretty tough becasue it doesn't have age categories attached to it. If one used the stats given, I think the half Dragon version would be almost on par with the epic dragons, and that is clearly not as it should be. So, I'm not sur ewhat is the best way to go with this. Buddha is the man on this topic, so I'll wait till he voices his opinion.