Let me start off by saying i am a bit of a purist when it comes to these smaller scale games.
In general it takes so few miniatures to field a decent 'gang' in AC'57 that it would seem silly to mix them up.
HOWEVER
i would also say that the scanario could dictate odd match ups.
i also am a sucker for storyline, so perhaps if one of your cops, spends his spare time bowling... and POOF you have a mixed gang...