Can I file an IG complaint against an E-8?

Of course you can but make sure you have all your facts in order

IG complaints can be filed against anyone for a LEGITIMATE reason. If what occurred in your circumstance was a misunderstanding over something that should or should not have been done, then you probably don't have a case. If you were disciplined and as a result had your feelings hurt, no case. Like others have said, you've got to have facts and it would be good to have corroborating witnesses.

Be careful, you may get backlash from complaining to the IG or EO.
Been there done that.

yes

FROM THE IG DESK. For the record, during my 30 years of Army service, I've held every IG position there is and was trained at the IG Academy in Washington, D.C.. A lot of water boiling over the issue of someone, "Filing an IG on (Name)" or similar. Following that is lots of advice on "possible repercussions" if (Name) finds out or someone in the CoC does. AR 20-1 specifies, "The Army IG system comprises commissioned officers, WOs, NCOs, and Army Civilians who embrace and exemplify Army Values, the Warrior Ethos, and the Army Civilian Corps Creed. These IGs hold positions of public trust, so broad experience, strong written and verbal communication skills, and impeccable ethics are key attributes that strengthen and build upon the viability and effectiveness of the Army IG system." An Inspector General is NO ONE's EMEMY! HE/SHE reports to ONE individual only...the COMMANDER (usually a Two-Star). It is the RIGHT of ANY service member to see an IG for ANY reason! It may be done in person (by appointment), by phone call, text, email, or anonymous - all forms of contact will be considered with equal respect. There can be NO repercussions whatsoever to any service member for seeking assistance from an IG. I felt it necessary to emphasize a point or two that seems to be confused in some minds out there. My first question will ALWAYS be, "Have you spoken to your Chain of Command before coming to me?" If not, that individual will be discretely sent back to their CoC with suggestions on who may best assist them there. My second focus will be to ascertain whether the concerns of a service member actually falls within my IG purview. If not, I will see that person is directed to the proper office for assistance. Be aware that some issues may involve legal ramifications and an IG has the authority to read a Rights Statement under 'Sworn' circumstances to a service member if necessary. Some advice I see given in other commentary is 100% on the mark...1) Be specific and chronological; ensure your documentation is correct and contains any and all FACTS necessary to support your issue (it doesn't need to be typed, just clear, and yes 'verbal' is acceptable as well). 2) Be composed and professional in both your writing and in person; don't come to any IG with your hair on fire or present a concern full of profanity and finger-pointing nonsense. 3) Understand an "Allegation" is exactly that - it's not proof of anything until a concern is looked into and finalized. 4) Understand that in most instances, the service member presenting a concern is usually not informed of the resolution (if any). The COMMANDER will always be the final authority on any IG concerns presented to him/her. I trust this acts to clarify the role of an IG with some particulars in reference to the IG system. JCH, Colonel, USA (Ret)

Run it by your unit Adjutant’s office, if it’s legit and by UCMJ, they’ll get it going. Don’t listen to the “Barracks/Sea Lawyers”, they’ll only make it worse!

Any soldier "can" file an IG complaint; but whether or not the IG will accept it has a lot of variables. For one, in my admittedly limited experience, was the chain of command approached first. I've had two IG complaints against me - one by a PV2 whose wife was a retired Major's daughter so you can guess how far that went - nowhere. The other a racial discrimination complaint by a soldier (SGT) I relieved of duty - that complaint was determined as unfounded.
The PV2 was ultimately given an Admin Discharge because he was literally, illiterate. The SGT was simply taken off road duty and assigned to his Company's Arms Room. What I'm inferring is that neither complainant's future in the Army was harmed due to their IG complaint; BUT certainly could have been.

Proceed through Chain of Command FIRST. Document every step of the way. Take notes, write everything up like you would an After Action Report. Having said that, I would caution that if the issue is something you can work out between the two of you THAT is DEFINITELY the right course of action; furthermore if it is not but is something you can find within yourself to live with (I would never choose to live with or turn a blind eye to anything I found morally wrong...technically or legally wrong sure I can live that shit but not morally wrong if you get what I'm saying) then just deal with it and move on. I found that MOST things I could just deal with and move on. I was involved in several IG issues during my career, one of which involved my own complain but only because unbeknownst to me the IG was in our shop and overhead me bitching about Finance screwing up my pay AGAIN...he got involved and the matter was settled quickly but I got an earful from the Finance CO because I hadn't brought it to his attention first...I cleared that up by explaining that was NEVER my intent and my complaint to my shop chief was simply overheard by the IG who was in an adjacent room. So cross your T's and dot your I's and tread carefully if you choose to proceed.

You can but quite honestly, the IG is a farce. I filed a complaint last year about an abusive O5 supervisor who had created an extremely toxic environment by literally yelling in my face whenever we had a minor disagreement (I'm a DA civilian) and his O6 boss who just sat there and did nothing to correct him. I had three eyewitnesses who gave sworn statements backing up my claims and I provided several other names they never even bothered to talk to or contact yet they somehow found my claim 'unsubstantiated'. The whole time they were doing the 15-6, they left me in the office under the two officers I'd filed a complaint against, both of whom retaliated by trying to ruin my career. Thankfully I escaped and moved to a better position in another theater. I have zero faith in the IG system, it is a complete joke.

You have the ability However this may not be the best idea in the world. You have not posted enough information to give you a responsible answer. I would 1st use your COC you may find several ways to solve the problem before going to the IG. If it goes to the IG your best support will be the COC who can make sure you have every thing right (t's Crossed I's dotted) they are after all your support group. Chances are if you are have problems at that level then so are others in the COC.