But seeing as that isn't a fortress but yet another planet (much less defended) I don't see why Xizor wouldn't just go mobile. His fleet of fast ships and pirate vessels would undoubtablly outrun the "millions" that Ulic's Krath Army has. Not to mention Xizor could use his intelligence network to predict Ulic/Kun's movements and conceal his own.
Expanding on this.
Kun might not even be able to touch Xizor after he's off Coruscant. The Fallen prince's information network will keep him up-to-date on Kun's fleet's movements, allowing him to be one step ahead and avoid ambushes, blockades, etc.
Also, the Falcon
had a hyperdrive mark 6, if I remember correctly. Even if, say, Xizor's fleet (made out of pirate ships that need to be and are meant to be fast) had an average of a hyperdrive 3 or 4, it could easily out-run the larger fleets of Traya or Kun.
It's going to be very hard to force Xizor into a fleet battle after he's out and about. Most likly the event will come on Xizor's terms: he'll use the element of surprise and the full force of the Black Sun.