Do you think writers should be only fiction or only non-fiction writers? I'd love to be both, but can one be a really good fiction AND non-fiction writer?
I think you can write anything you want to! If you have the ability to do both, then there's nothing to stop you. The biggest issue is probably just a business concern. If you become a very big fiction writer, your book company may tell you, "More fiction! More fiction!" because that's where your fans are. So, you'd have to make some career decisions and decide what's most important. Good luck!