An intersting ideal. I'd say at the minium, the Vorlons and Shadows would auto war them. With the new DLC it might be possible to even turn it into a "war in heaven" style event. Offering younger races the option to join the Shadow/Vorlon alliance, the invading Thirdspacers, or try to survive on their own. I know that sounds weird. But think as much as they hate each other. They would see this as a power in their league poaching on their "preserve" of young races. I really think they would put aside differences just long enough to band against the interloping Thirdspace Aliens.