I'm guessing that you mean CvS2, right? <IMG SRC="smilies/smile.gif" border="0">
I think it's all determined by the number of GPS you have after the "final" match. If you have 1000 or more, you'll be able to fight a boss. I've already fought Rugal and Akuma once each, but I'm not sure if it's the Shin versions or the God versions at all.