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

During Desert Storm, I was assigned to lead patrols on the perimeter of our camp due to previous training and experience. I was an E-5, yet would sometimes get E-6, and even E-7, personnel detailed to me. I respected their rank, of course, but they always let the rest of the patrol know that I was in charge, and that they would obey any lawful orders I gave. This is exactly how your E-6 should be acting. Good leaders are good followers, too.
I would suggest that you bring the matter up to your actual chain of command, as his behavior is likely causing disruptions to the work flow, as well as creating friction in the unit. I wouldn't confront him directly about it, if I were you, as it could lead to more problems.
Personally, if I were filling an E-4 slot as an E-6, I would enjoy the vacation from the normal responsibilities of a staff NCO and just do my job and chill.

A SSG is a SSG is a SSG. So yes you consider them a SSG. I put a 12C SSG in the S1 for a time because the section lacked discipline and we were short, the admin SGT was technically savvy and did well at the Admin tasks but was overwhelmed by the additional leadership tasks required to run the shop (normally an SFC).

What I noticed with our NG round out unit in 03 was that in some cases the "go to guy " wasn't the guy with the most stripes and that the unit took folks from all over to bring it up to strength and folks were working in MOSs and slotted in grades that they hadn't been in, in years just to fill the unit. They did a great job for us but it took a minute to figure out who was who.

Point is; regardless where the SSG is slotted on the MTOE if they have the MOS as a secondary or third the Commander and 1SG can utilize them where they need them. Even if they don't have the MOS but there is a need that they best fill they can go where the Commander and 1SG need them.

CSM Richard StCyr pretty much hit the nail on the head. Regardless of the MOS, the rank is still the rank. That does not mean that is the best person for the job. I understand you may not like this but it is the challenge of dealing with difficult people. This is a leadership challenge for you to learn how to earn the respect of someone who outranks you, even if you are upset with them. That aside, when that SSG is acting as a 92Y and if that is not his lane; smile, be professional, and move out. Your best authority comes from the Army Regulations (AR 710-2) and DA Pam (710-2-1) and when an order violates that, point it out and move out! You appear to be frustrated and looking for a way to negate the leader that was placed above you. My career was 30 years and at least 27 of them I had people that outranked me that were in some cases, clueless. Regardless of the "power" I may have had, those people still had the "Authority". Respect, Respect, Respect, PERIOD. Learn from the experience and just don't repeat bad behavior. I get it, easier said than done. Been there. SGT McMasters, Deep Breath...Roger that... Move out!!! Airborne, All The Way!!!

Chief