Can protestant league form if Reformed or Hussite are electors

[Octopuses]

Or does it always have to be a protestant elector?

[grotaclas]

Have a look at the wiki: https://eu4.paradoxwikis.com/Religious_league_events#The_Evangelical_Union

[Octopuses]

Nice. That will make Bohemia achievement very easy.

[Dmformom]

Yep. Just be careful not to convert too much early on, Hussite nations in the HRE seem to be weighted against joining the league war which can make it a bit difficult.