The SAS was formed in 1941 as a commando force operating behind enemy lines during the war in North Africa and disbanded in 1945. In 1947 the Artists Rifles regiment was remodelled as the nucleus of the reformed Special Air Service which has become the model upon which many other countries have based their own special forces units.
the clue is in the name, we all know SAS stands for - Super Army Soldiers, so that means they must come from a variety of ARMY regiments. As long as the soldiers are super and hold a minmum of one of the basic superhuman abilities like .....invisability/ flight/shapeshifting they should stand a chance of being able to serve in "THEM".
A little known fact of the joining procedure for the SASS Is your ability to have a good nickname,like Legs...dinger...or johnny 2 hairdryers, I think Brummie is also taken, but as long as you can think of something hard sounding it stands you in good stead.