Two questions from a new player.

[Bahnzo]

What's a good Custom Battle to start learning the game? Playing Union. Something that's hopefully relatively balanced so I can learn the game before I progress to campaign. 

Is it worth installing either of the mods right away, or should I play a little first?

[pandakraut]

The UI mod could be used right away if you want. Mostly just bug fixes and quality of life changes.

J&P is more it's own experience. While you can jump straight into it, it's aimed more at second playthroughs since it changes up so much if the game.

I don't have any recommendations on custom battles, I just learned by playing the campaign.