T-Rex co-exists fine with Raptors and Deinonychus packs. Every once in a while they face off for half a second, but haven't had any fights or deaths. Though, I did find out that tranquilizing a Deinonychus in the same pen as the T-Rex will cause the Rex to come kill the tranquilized dino, which I wrongly assumed was not a possible game mechanic (I assumed dinos were magically protected while tranquilized). I therefore assume that a large carnivore will eat small carnivores if they are tranquilized in the same pen. Interestingly, the same is not true for the opposite script, the Deinonychus' left the T-Rex alone when she was tranquilized in their pen...
That's also the case with Deinonychus and Metriacanthosaurus. My Metriacanthosaurus killed one of my Deinonychuses while the latter was tranquilized.