It is later revealed in the second book Pegasus and the Fight for Olympus that they are a benevolent race and they only attacked Olympus because the Gorgons (Euryale and Stheno) were controlling them.
They cannot be harmed by anything except for Olympian gold, which poisons them.
In the third book Pegasus and the New Olympians, it is revealed that the CRU had taken DNA from the dead Nirads in their possession and mixed it with Olympian DNA to create Nirad clones. Because they were part Olympian, these Nirads were able to speak English and communicate with those who couldn't speak the Nirad language. Despite being clones, they could still telepathically communicate with Prince Toban.
The Nirads seen in the first book are all gray, four armed, and hairy. In the second book, Emily learns that there are many other colors of Nirads. There are also lilac and orange Nirads. Tange and his brothers were orange Nirads. Queen Segan and Prince Toban are pink with some marbling. They are also smaller than the other Nirads.
The Nirads are ruled by a queen. The queen is able to silently communicate with all of the Nirads. The original queen of the Nirads was destroyed by the Gorgons right when they arrived. This forced Tange's daughter, Segan, to become the new queen. The Gorgons would turn young Nirad children to stone if she didn't obey them.