You can bring witnesses, I have also used testimony from witnesses by phone. The only caveat I would say is that you want to have your attorney prepare them for testomony (review what they are going to say and see if there are any issues with their testimony). There is a saying (not 100% true in all cases, but it is common wisdom) that an attorney should never ask a witness a question he doesn't already know the answer to.
If your commander is testifying, the focus should be on how your conditions impact duty performance. As for what to expect, they will be sworn in, your attorney will ask them some questions, then the board will ask any questions they have. That is about it.
Having witnesses testify is not the norm, the bulk of evidence in cases is medical evidence and the Servicemembers testimony. I would say, every witness should have a specific purpose for testifying. You don't want a bunch of witnesses saying you are a great guy. It should be tailored to the specifics of your case.