While it is true to say that humans are the most dominate life form in the known universe, it would be false to think that humans are all alone in space. Indeed, there are many species of alien life in the universe, and humans make up only a small fraction of that.

Aliens come in all shapes and size as well as a bewildering variety of colours and smells too. But the humanoid form of two arms and two legs is the most common. Indeed, the Cylons found that this form was the most versatile and copied it for their robots. Why this is so is not clear and has been a matter of debate for yahrens and probably will be for yahrens to come as well. Several theories have been put forward, including one that states that all humanoid life springs from a common source in the distant past, but there is no proof to substantiate any of these claims.
Inter-breeding between differing species is generally impossible - even amongst similar humanoid races, but that obviously does not stop some from trying!

Despite the large amount of aliens there are in known space, the actual amount in the Colonies themselves is actually rather small. Whilst the Colonials do greet all new races with open arms and a peaceful intent, and aliens are generally made welcome wherever they go in the Colonies, the truth of the matter is that the Colonials would rather meet the aliens on their own worlds rather than bring them back to the Colonies. Thus the number of actual aliens living in the Colonies is very small. In addition, there are a number of designations that are banned to aliens - they are not allowed to enter the Colonial military for instance without several background checks. In fact, it is true to say that a lot of Colonials will never meet an alien in their lives, although they will have heard of some of them.

One main reason for this is that the war with the Cylons has made the Colonials slightly wary of aliens and outsiders. Those that are not known could potentially be working for the Cylons and thus could be the one to lead a Cylon task force back. This is obviously not good.

Alien races should not be ignored in any campaign. Whilst there are not any that can rival the Colonial and Cylon species in known space for size and power, there are plenty that can spice up any game. In this respect, GURPS Prime Directive is a very good book to have as it includes a long list of advantages and disadvantages that aliens can use.