Translation:The general is the commander of the army.
Using "in" instead of "of" is not accepted. Although I think the "of the army" construct is more natural in English, "in the army" doesn't sound odd to me and it follows the Greek grammar a bit more closely. :>
That would be Η κεφαλή/Ο επικεφαλής του στρατού which may mean the same thing, but it's not the same word. ;)
Is that acceptable in english? If so, then yes, it could be used, but i had in mind that chief was mainly used with tribe, team, etc