The Sirians are another race I created, based on the little copper-skinned guys in "Journey to Babel", and on the infamous venus-drug in "Mudd's Women". How can they go together? Here's how:
Sirians are a race of small, hairless humanoids. Skins range from copper to light gold in color, and have a metallic sheen. Their physiology is somewhat Vulcan-like, in that they have green copper-based blood, a high heart-rate, and rather low blood pressure.
Their homeworld, Sirius IX, is a harsh planet. It is dryer even than Vulcan, with high gravity, a climate that alternates from extreme heat to extreme cold, and a dense, stormy atmosphere. The extreme temperatures are made even more exaggerated by Sirus IX's long year, equal to one Earth century, which leads to "seasons" approximately 25 years long.
The Sirians have evolved to meet these conditions, developing a small stature to save food and water, high endurance, and a tenacious, determined mentality. Sirians are practical, tough-minded, level-headed, and basically cooperative. They do not waste resources on the wars and conflicts which distract other races. They cannot afford to do so.
Sirians are instinctively monogamous and mate for life. Given that their lives are long, there is often a need to maintain the passion in marriages which endure for centuries. It was this which led to invention of the Venus drug, later to be misused by Harry Mudd.
The most important Sirian social institution is the underground city. From their earliest history, Sirians have burrowed to avoid the extreme temperatures of their homeworld. In time, the burrows became mines, workshops, and communities. Sirian cities stretch on for miles under the ground, with elaborately carved stone passages, galleries, and chambers. Such buildings as appear above ground are low and functional.
Most Sirians will gladly die for their city, which is the center of the social, spiritual, and economic lives. Most cities are ruled by democratically elected councils, though a few cities select their rulers based on merit.
The planetary government is a decentralized alliance of cities, with little power beyond the authority to arbitrate disputes, maintain trade, and represent Sirius IX in the Federation.
Sirian "style" is mostly unpretentious and comfortable. Clothing is soft and warm. Art is intricate and mostly abstract. Music is mellow, soothing, and instrumental. Food is rich and hearty, featuring thick stews, filled dumplings, and hot drinks.
Sirians' small size makes them rare in Star Fleet, though they make excellent engineers. They are also common as merchants, colonists, and administrators. There are very few Sirian rogues or criminals.