Hidden Bosses can be used once they have been defeated. The conditions of the Bosses to appear are to fulfill two out of the following three conditions. If you play with a Capcom groove you will get God Rugal, and if you play with a SNK groove you will get Shin Gouki.
Achieve over 1500 GPS by the end of the game.
Defeat the Middle Boss (Geese or Bison/Vega).
Achieve at least one Finest KO.
The Custom Groove option appears once a normal Boss has been defeated (normal Gouki or Rugal). Normal Bosses appear by fulfilling two of the four following conditions.