The problem is not the six suns but the constraints on the planet and moon:
* that the planet's orbit be stable over thousands of years. Millions or billions, if you want life to evolve.
* that the moon stay invisible at all times -- never be illuminated enough by any sun to be visible.
* that the moon be wide enough in angular size to eclipse one sun for over a day!
If you read this paper, you see they settled on a moon the same mass as Kalgash but with the density of Saturn! How could such a system possibly arise?