When you say realistic 20-21st century, does this mean the aliens need to be reflective of modern social issues in regards to what sort of cultures and forms they can take?  Can aliens have a very distinct tech base from the Llvyr Empire or must all the tech be similar regardless of home world origin?

And also, do all species have to have a history with the Llvyr Empire as clients/former clients or can you have things that developed outside their whole continuity?  The need to be part of the UA seems to imply the former and I am not 100% certain on what players can be in this RP.