Hey! Great that you are going to be joining in :)Yep! Anything standard is allowed and any tools like shaderforge, playmaker, probuilder and everything is allowed.It's mostly large sections of code/ art assets that need to be original.
Can we use a song of another game ost? Or royalty free music?
Yep! Go for it :)
By another game i mean, Hyper light drifer, it is okey?
you shouldn't us copyrighted materials. That would normally include music or any assets from another game.