Use multiple IDs seperated by comma?

There is a pro version of this plugin that will allow you to use multiple product IDs as you are describing. You can get it by visiting: https://surpriseazwebservices.com/plugins/conditional-woo-checkout-field-pro/

Similarly, I would like to use this for product categories rather than by item. Is that possible?

@petes42 no not at this time. Only product IDs are used to control whether or not a field is displayed. Product categories is a commonly requested feature, which is being considered in the future for the Pro version.