Every species is allowed to travel freely within the Empire and use the stargates and star highways as they please. To be able to use the galactic highways, the star race needs to be interdimensional and evolved enough to be able to nano-travel.

Every species is allowed to travel freely within the Empire and use the stargates and star highways as they please. 

To be able to use the galactic highways, the star race needs to be interdimensional and evolved enough to be able to nano-travel. 

It is my understanding that there are no 3-D races in the Empire, because you need to have access to more dimensions than just the third to be able to interact with the more evolved beings within the Empire. 

However, a star race is normally not 'stuck' in one density or dimension for very long, as they are free to evolve without interference; it's just we humans and some other star races in a similar situation as ourselves who are 'stuck' in 3-D for a much longer time than we should. 

This doesn't mean that star beings who are multi- and interdimensional don't have bodies (although there are some who don't), but they have learned how to split their Fire and travel across space and time, independently from their 'physical' bodies.