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

If its not a criminal matter don't do it. You will pay dearly.

Do you need to? Chain of command first/////

Perks of the military...the bootest E1 can file a complaint against the SEL or CNO (whatever your branch is) as long as it's justified.

Be sure to use the chain of command first and if they don't help, the IG and your Congressman or woman, should get a letter from you telling them your problem. Also if you have an EEO the are a service that can settle problems or put evidence
together

Remember the Chain of Command. Go the the Installation IG and file the complaint.

This created a lot of reaction, what is the complaint?

You can file a complaint against any rank. But, have you tried to resolve the issue at the lowest level and within your chain of command before escalating it?

There is not a rank cut off for an IG complaint.
If it is a valid IG complaint you will see results.
If it isn’t and your are just being salty, you won’t see results.

Depends on circumstances of which you have decided to be nontransparent.
A substantiated IG complaint can be filed on anyone.
Footnote: use chain of command and have your ducks in a row filing on a senior noncom...

I would probably approach a JAG attorney or seek advice from the Chaplain, before taking on what could turn out not like what you anticipated. Have you considered approaching your chain of command on the issue you are having w/ the E8? In any case be very sure you have your facts straight.