When the Stanford instructions explicitly state in bold:
Mixed category portfolios are permitted but not recommended.
I think you found your answer
Now, you might be the most promising artist in the world, which means s why they don’t say that it’s prohibited. IDK. Or perhaps you have an overly inflated sense of your own talent. Again, IDK
But given that you’re already at a disadvantage as an international, and given that they like applicants who can follow instructions, are you wanting to risk it?
So to be clear, unless it’s the best portfolio they’ve seen, yes, ignoring instructions will hurt.