This may be a real basic question, but what are the rules for swapping chainrings from crankset to crankset?

Are all 130 BCD chainrings compatible? All Shimano? Are there differences between double and triple?

Basically, I have a Shimano 600EX crankset, with biopace rings. I would like to replace one or both of those rings. It has a 42t small ring, and I want a 39t ring. I would prefer to get rid of the biopace at the same, but that is not really required.

It would seem like I could use any 130BCD chainrings. Is that right? Even later 9 speed rings?

And what about triple rings? I would some day like to get a ultegra 6503 9 speed triple crankset for my wife's bike, but again I would prefer a 39t middle ring, and most of those come with 42t. Can I use any ring, or are they triple specific?

Jared