While we can add food values to non-tfc food, but we can not specify what food category it is part of.
This is a problem when it comes to soups and such, as they specifically check for categories (for example soup checks for Food.Category.FRUIT, Food.Category.VEGETABLE, Food.Category.COOKED_MEAT, Food.Category.MEAT).

This means currently any non-tfc food would not work in soups (and im guessing sandwiches/salads, but i didn't double check the code for those).

Meta Info

• TFC Version: 1.5.2.152
• Mod compatibility

Yes you can, as the category is simply an ore dictionary name of the category, prefixed by the word category. (categoryFruit, categoryVegetable, etc.)

Ahh that's really good to know, wasn't aware of that feature, my bad!