Since the Dark event is coming up tomorrow, I bet a lot of people are focusing on light. Not sure about Dark. I'm not sure if there are a lot of people who consider Dark/Light to be best from what I've seen, but I know they have a small advantage over the basic elements, and a large one over one another.

I think if you can, consistency in one element is probably better than anything in the long run.

I think for now, a mix of elements is perfectly fine so long as you're just working toward increasing the strength of your teams (Weps, Eidolons, Breaks).

No idea if that helps you or not.