Relics doesn't place requirements on what you can and can't choose in your force except for making units more expensive the more you take.
This means that there is no requirement for you you take a commander in your force.
However, there is a section of the rules called 'Command'. If a unit is out of command, it's available actions are reduced and if forced to take a morale check, it is harder to pass.
So yes, you do not have to take a commander if you don't want to but not doing so will put you at a disadvantage.