^ Huh? Maybe I'm reading that wrong, but are you saying that you can only use the PK Delcatty and not the R/S one? Because that's incorrect. If a card has been reprinted in a modified set, then all the previous ones before it are legal as well. Just know that if there is a slight change in text flavor, you go with Most Recent Printing Rule. Like with Metal and Darkness energies. You can use old ones from Neo Genesis, but they read like the current Metal and Darkness energies (only working for Metal and Dark Pokemon, respectively).
For an easier example, you can use Rare Candies from Sandstorm because Rare Candies are legal and they have the same text as the Emerald and Holon Phantoms cards.