The only rule I have FatHarry is that I have no rules!
I have a couple of novels sitting in drawers (they are definitely too dire to inflict on the world!) and with each I had no idea where they might go. Some were a loose idea. Some were a powerful person that I wanted to write about and things would kind of 'happen' to them..... And I never worry about the genre. Many cross over into one or two genres anyway. I don't like to be defined so narrowly!
A lot of my idea's are those 'what if' ones....ie, what if a life-changing event occured to such and such person at such and such time in their lives? And I go from there.
A novel I have recently submitted to a publisher is different in that I
did know the story from beginning to end. I could see the whole thing in my head clearly. I felt I could see and knew the characters and could almost walk down the streets where it took place. Perhaps this clarity is why I finally felt this book reasonable enough to submit.
Good luck FatHarry, hope you overcome your lack of confidence. Your post alone is well written and concise, so I imagine your writing is too.