What are the rules for a space marine commander?
Was this Space Marine Commander the miniature you saw?
If so, at least according to the description on the website, it appears you'd be treating this as a Captain for rules purposes:

Space Marine Commander
A Captain leads each of the ten Companies of a Space Marine Chapter. They are second in experience only to the Chapter Master and each is a warrior so deadly that he will rarely meet his match. Each Captain is an inspirational and determined leader, able to coordinate the Space Marines under his command whatever the opposition.