As I recall, the Ancients monologues also state that the ancients had a habit of wiping out races they encountered. Though there may well be races way, way outside of GTVA space, it's going to take many years to explore the space previously occupied by the ancients, before reaching any systems that have any possibility of housing a sentient species not wiped out by the ancients or shivans. I wouldn't say it's totally impossible, as a galaxy is a big place, but you're talking about centuries of exploration, travel, and dodging/fighting shivans. Of course, something could be a single jump outside of Regulus, but it seems unlikely.