Where can I research the topic of talking to multiple career counselors? Is this against any regulation or policy?

Your Career Counselor is attached to your Battalion. You have one Career Counselor who is authorized to work with you. If there's an issue you can go see their boss at Brigade. It's not a matter of giving them the courtesy. Would you go to different Platoon and ask their PSG if you can put in for leave? When Soldiers are Career Counselor shopping it means they don't like the answer they got from their own Career Counselor, and they're looking for someone to give them a different answer. I'm not surprised your Career Counselor is making your PSG come with you. Because the next thing you know you have Soldiers emailing HRC and school houses and getting different answers and it's a huge headache.

SPC (Join to see) , while not technically against a policy or regulation, it can cause you heartache later. As long as you are doing it to get a different perspective, you still need to work through YOUR Career Counselor to action anything. Your generation of Soldier never trusts one source, so I get why you did it. Some of us more senior people get livid when you do it, but I have just accepted it. As long as you follow the right processes after getting a different perspective, you will do ok in your career.

Now, as far as the counselor only wanting to talk to you with your PSG, you brought that on yourself. You broke the trust of a lot of leaders by going outside the unit (which it sounds like you brought up that you did that when trying to get your way), and now the counselor wants one of your leaders there as a witness to what you are being told. It sounds totally reasonable and hopefully you learned something from this.

You went to a counselor from a different Unit? That's odd. Generally when folks it sends a red flag because if they are going outside of their bubble there is generally a reason. Look at it like you are 8 yr old and asked Mom for Ice Cream. She said no so you go to Dad and ask the same question because you really want ice cream. Your Counselor wants your PSG there because it is CYA on their part. You broke protocol and he wants to know why but he also wants a third party on hand to help keep things grounded.