To make matters even more confusing, at one point in the comics run, each piece of Red K could affect each member of the super-family (Superman, Supergirl, Krypto, et al) one time, and after that it would have no effect on that member.
I recall one silver-age story (sometime in the 50's - I read it in a paperback reprint, honest!) where a piece of Red K gave Superman a third eye in the back of his head. He used it to increase the effectiveness of his heat vision to defeat Braniac. That piece could never affect him again, but he got rid of it so it wouldn't affect Supergirl (I think he threw it into the sun). The irony was the Braniac was the one who exposed him to the Red K in the first place.
For what it's worth, my advice is to use however you want, as long as you're consistent within the parameters of the story lines you develop.
ETA: Drat. Lynn beat me to the punch.
