Rex had been planning this very moment when the pack challenge was issued.
He knew that the death—of the Fifthborn was a big deal, all races, would be saved from total destruction under a tyrant, but that didn't change the fact that some forces were still at odds with him and his ideals.
Vampires became a passive force now that their princess was essentially betrothed to him.
Calidora's obsession made that possible, but they are still an opposing force.
If a chance present itself, the Vampires wouldn't hesitate to strike.
Demons could also fall into the passive force category—Elder Tilrith, and Rex were in a better relationship compared to the rest. She helped him several times and even their last talk didn't end up in a brawl.
Now that the Angels appeared, the Demons at the very least have their attention divided.
Only the Shapeshifters were in a gray area.