If I understood your idea correctly, you are using alternative-specific to prevent attributes that do not change (such as size) to a level. Conditional display would be a good approach if you want this fixed size to still show up in the size attribute row. Let's use your example of product and size.
If you prohibit Product C from showing with the size attribute, you would see a 0 level (prohibited level) option in conditional display when pairing the product attribute and size attribute. Now when Product C (Level 3) shows with the size 0 level (prohibited level) whatever you put in the display text will show up when that combination is prohibited.
You should keep in mind that you will not have a utility for this 0 level and Product C with the fixed size level test makes it so that you cannot see if Product C would be more or less preferred in a different size.