What should I consider an E-6 who takes an excess E-4 slot for a deployment?

I think that he is still a SSG and should be treated like one.
If he is poaching out of his lane in HQs, that is First Sergeant business to fix.
I can see scenarios where command wants this guy to check in on supply now and then to make sure you guys are on track and doing things the right way.

If they are wearing the rank, that is what they are. Your leadership has to clarify the individuals role in your area. If he is asserting authority, it may have been determined. In any case treat them with the appropriate respect and inquire as to there role.

It happens frequently that a trusted NCO is put skmewhere to correct leadership issue, the management, training, mentoring and discipline of a section, office or platoon. They do not need to know the MOS to lead it.

I was in line infantry units and at times had E-7 try and bully my with their rank to get special favors dealing with supply and HR’s. I would politely hold my ground and refered them to the commander. Supply is commander run and only personel assign or authorize should be into supply business. I had a 1sg that constantly would pull my guys for details or pull them to help out another platoon and I had to go to the commander about it cause it was causing us to miss important deadlines. Use tact and talk to the commander about it

First of all experience and holding the MOS is 2 totally different things. Anyone can be s HR holder but not everyone is a 92Y. Let your commander know what’s going on and have that E-6 stay in his lane

Those being in charge are rank or position. If your position is higher, than he’s your subordinate otherwise, follow your military bearing.

I had a SSG transfer in, we both were not in our MOS. I had been in the section for two years. The new SSG came in and without introduction decided upon himself to change things just be cause of his rank. Well you either rub them the wrong way go to the CSM who we worked for or let him fall on his face. I went to CSM and explained, my position. Well the SSG had not been told to take charge he had been told to stick close and learn. CSM and I were pretty close, close as we could be. He then said let the SSG fail. So I made notes a lot of notes.

Sometimes you either rub them the wrong way or allow them to fail on their own. Either way make notes!

Remember, the job has nothing to do with the rank...even officers take out the garbage once in a while...does that make them E-1 ?? NO !!!! Be thankful you have someone of mid-grade NCO status working with you...BUT, if the NCO starts to "jump bones", quickly clear this up with you immediate senior officer...God bless ya !!!

He's an E6. Whether he's in an E4 slot or not, he still holds the rank. I would pull him aside and talk to him behind closed doors about staying in his lane, for sure, but at the end of the day, he technically has rank over you.