One or Various Niches?

You are asking too much irrelevant questions. As long as you do not create assets for overrun themes, and as long as you are keywording and assign good titles, and as long as the quality of your assets is good, you will do decent sales. The more assets you have in your portfolio, the more sales will happen. 

Due to my background as a photographer (portraits, fashion, etc.) I tend to concentrate on doing what I like doing most, which is, obviously, portraits and fashion. But I also know there is a limited need for same. So I'll do food, animals, people in professional settings, backgrounds, wallpapers, tech, whatever. My fashion stuff does sell, but not as well as the latter material. So yes, diversify, but keep things interesting for yourself rather than attempt to fill a niche that may not even be of interest or that is already over-saturated.

Diversification is good, allowing you to reach more Buyers.