How to find a good business partner: "When I was searching for a co-founder to help me run Okta, I came up with three rules: (1) don’t go into business with a close friend (2) don’t settle for a stranger and (3)choose someone you don’t mind spending as much time as you do with your wife "

Those are good rules. 

It's really hard to find a co-founder so the number 1 rule should be: It's not easy!

So I am thinking now - does "cultural fit" and "diversity" are the opposite of each other? On one hand, people want to work with somebody they are comfortable with, on the other hand, they need complementary qualities in the team...  Probably, diversity should not be a goal in itself.

Culture is a product of the values of the founders. 

If the founders value diversity, so will the culture.