Adding custom leagues, need some tips.

I'm starting a worldwide journeyman save with ~20 or so nations loaded. My knowledge outside Europe and the USA is practically zero, so I need some advice.

First, where do I go about finding custom databases that make previously unplayable leagues playable?

Second, for this save, which additional leagues should I be looking at loading? I'm trying to cover as much ground as possible, preferably leagues with a half decent reputation and talent pool, perhaps Japan and the UAE?

Lastly, for those who have added unplayable leagues - does this infringe on anything else in-gams? Will my real name fix file still work? Anything to be aware of with database size and setup?

Appreciate any feedback at all, just want to get this right.