There's this, but they are all mixed together.. Is there a list somewhere that has all the friendly challengers? (and only them)

While it isn't only the Friendly challengers, the sections in that guide marked Secret Opponents does have all the Friendly challengers. The mechanics to get the other Secret Opponents are similar to how you get most Friendly Challengers.

This guide tells you the loot they drop but also lists how to unlock each Secret Opponent all on the same page. [Url]https://docs.google.com/spreadsheets/u/0/d/1PSNYR8fYYJ7VA-DOBWTFTlVL0QohQHG1jnY1qMBn1Tg/htmlview#[/url]