Re: Your first question
The way I handled this is to describe availability in the product description (Note that Yellow is only available in size Small), and then to validate the attribute combination when the user presses "Add to Cart." A description of how to do this validation in Javascript is given in
http://www.thatsoftwareguy.com/zencart.html
Catching the dropdown box change to "Yellow" and repopulating the size box with just "Small" could certainly be done in Javascript. However, it would annoy users who selected "Large," then "Yellow" and then pressed "Add to Cart;" they would see a "Small Yellow" in their cart and wonder why. The validation route is probably the better one.
Good luck,
Scott



