An new employee comes in and on the very first day, starts questioning other employees about their pay and then goes to the boss to complain how unfair the pay process is.
You truly think that's a good idea?
Whether the NLRB would agree with jal that your questioning the other employees about their pay is protected, is open to debate. While I don't disagree that it could apply, as I recall there were some fairly strictly defined limits as to what was and was not protected and I don't think we have enough information here to say if what you did falls within those limits or not.
But this: So out of asking, I would be able to confront the boss, or coworkers who are paid unfairly out of my discussion with them, and ask them why Johnny Doe had no experience and started out higher, or Susie Q has more experience but started out less is in no way protected, is outrageously inappropriate, and I would expect you to be fired on the spot. I can promise you that the NLRB would have no problems whatsoever with that firing.