Is it possible to have a mod that will enable friendly fire so that all projectiles hurt everyone, regardless if it comes from players, IDPD, enemy mutants, or bosses?

You can - it is a matter of setting team variable to different values for each entity of interest. See PvP mod for reference and try drawing the variable above entities/projectiles to understand how it is used.

I suspect that giving each enemy a different team would mostly result in them killing each other on accident - the AI will still pursue the player(s) and not be mindful of allies.