I'm going to try a different approach here. When you mentioned "Alien Races" I thought of two books, Gurps Aliens, and Points in Space 1: Starport Locations ( http://www222.pair.com/sjohn/points.htm ). There are two approaches here that are worth looking at.
Gurps Aliens was more about character generation than in depth background. So lots of stats on how to build the darn things, and what was the borderline normal. Each alien race was a few pages long and unless two races specifically had a history I don't remember a specific setting, or more than a couple of sentences regarding interactions.
Points in Space isn't even a aliens book in the traditional sense, but rather used aliens, in a totally statless way, to flesh out the location details. The author highlighted the important details, and showed contrast, to the point I could use any of the races mentioned, but it is a statless book.
All this is going a long way to say that, while the Traveller method and level of detail is perfectly valid, I think another valid approach would be to be to go a little more minimal. Enough rules material to help folks make characters/npcs without reinventing the wheel, and describe the truly alien parts of the aliens to give GM's a quick grasp, and inspiration for using the aliens. That leaves room for your consumer to implement them however they like in their own setting.
Just food for thought.