Can I use Sceptile's "Nurture and Heal" ability to attach a 
Energy to a Pokemon that doesn't have any damage on it?
Yes, you can.
Nurture and Heal
Meganium ex
- Type
- POKEMON
- HP
- 150
- Energy Type
- Stage
- STAGE2
- Retreat Cost
- Weakness
- x2
- Resistance
-30- Rarity
- RARE HOLO EX
- Illustrator
- Ryo Ueda
- Set
- Unseen Forces (UF)
- Number
- 106/115
Abilities
Nurture and Heal
Once during your turn (before your attack), you may attach a Grass Energy card from your hand to 1 of your Pokémon. If you do, remove 1 damage counter from that Pokémon. This power can't be used if Meganium ex is affected by a Special Condition.
Attacks
Razor LeafPower PoisonDiscard 1 Energy attached to Meganium ex. The Defending Pokémon is now Poisoned.
When Pokémon-ex has been Knocked Out, your opponent takes 2 Prize cards.
Rulings (3)
If I use Sceptile's "Nurture and Heal" ability, is that an extra Energy card attachment or does it count as my one attachment for the turn?
It is an extra Energy attachment. You still get to attach one other Energy card as your normal attachment for the turn.
Nurture and Heal
If I attack with Sceptile's "Power Poison" attack, do I have to discard an Energy card if I don't want to Poison the opponent's Pokemon?
It is not optional. Even if the damage would be enough to Knock Out the Pokemon, you still have to discard an Energy and Poison it.
Power Poison