I use a 55mm (4.5x6) for about 80% of the time, but for those other 20% I do have what it needs with me. And sometimes I really want to use 6x9 or 6x6. Even use digital sometimes.
I did found out that different formats on the same outing is not a good way for me. Or digital and film if they do not cover a different goal. But different focals is a must, it just feels natural.