So, the Dragon Herald (Bard;Kobold) archetype has a couple abilities based on the energy type of the dragon the herald chooses as their patron. The wording of the archetype allows for any type of true dragons to be chosen as the patron.
The two abilities I'm wondering about are Coat of Arms (Ex):
A dragon herald gains energy resistance equal to her dragon herald level against the energy type associated with her dragon patron. This resistance stacks with any inherent racial resistances, but not with magical or temporary resistances.
and Rebuke Foes (Su):
At 12th level, the dragon herald gains the ability to rain down her patron's wrath on her enemies, dealing energy damage of the appropriate type equal to twice her dragon herald level (Reflex save for half). She can target one enemy within 50 feet for every 4 dragon herald levels she has. This effect occurs once per round for as long as she continues her performance.
Your standard metallic/chromatic makes sense (Acid, Fire, Cold, and Electricity are pretty straight forward), but there are some other true dragons that are less straight forward.
The (Imperial) Forest dragon has a breath weapon that does piercing damage, and the Guardian Dragon does con damage from poison. How would you handle these energy types? They're technically true-dragons (I'm only focusing on paizo sources right now), but I'm not sure if poison would count as an energy type, and I'm pretty sure piercing wouldn't.
The obvious answer would be "Just don't use those dragons as your patron", but if I was going to accept that, I'd probably not be posting it here.
Would these dragons be legal to use as patrons, and would a Dragon Herald get con damage resistance or piercing resistance, respectively?