If Porygon2 has changed a Grass energy to Lightning, would it be fair to say that Venusaur could not Energy Trans it (for the rest of the turn)?
Correct.
Source: WotC Chat (2002-01-17)
If I have 2 of the same Pokémon in play that have a Pokémon Power [or Ability], can I use that power twice? What powers wouldn't be useable twice, or more for that matter?
Sure, depends on the particular power though. If you had two Muks for example, then only one power is necessary.
Source: WotC Chat (2000-05-11)