If a hero went inside a colossal dragons stomach and started stabbing him in there would the dragon be able to use summon monster within itself in order to fight the hero that is in his stomach?
No. As per the SRD, under Conjuration:
A creature or object brought into being or transported to your location by a conjuration spell cannot appear inside another creature or object, nor can it appear floating in an empty space. It must arrive in an open location on a surface capable of supporting it.