1. Early on it's better to just focus one or two teams because you're still new and limited to resources but when you've gotten far enough or reach your current peak for that element, you always have the option to level the others. There are no cons to leveling each team other than using a bit more time to do so, which anyone will have if they have reached the peak of their current element and have nothing better to do but wait for future content.

2. Miracle tickets are not required, they simple help in building up a team for those willing to spend for it and outside of rerolling, is the only way to guarantee a specific SSR and that one SSR can make a big difference for some people.

3. No you don't. Aiming for -50% atk and def down is a good goal to set but it's not required to complete anything. Just having more makes it easier on you, at least for most fights. You can complete most, if not all, content without it needing to reach the cap, simply having 'a decent amount' is good enough.

4. I've only heard it's a feature that requires the usage of many different himes or something, that's probably why they say those who stick with one element will have trouble. That's just a guess. Someone else can inform you better.