Nathan covers the niche thing well. Cherry Adair said something that struck me two years ago at the Emerald City Writers Conference in Seattle. People ask her: can I write in multiple genre? Her answer: Yes, but if you want a career in writing, pick one specific genre and have that be your money-maker that you can count on. Then you can branch out into other things.
Sort of the way Chef Ramsey tells restaurant owners that they need one meal on their menu that is their primary money-maker.
I've got two main genres: science fiction and thriller. I can see from sales that there isn't that much crossover among readers between the two (for example, UK Kindle sales are 90% scifi). If I had to do it all over again, I'd have focused on one thing. But, now that I have such a huge backlist, it's okay. The issue now for me, is which genres and series do I project forward?