Dear Sirs,

What are the rules for using articles with groups of people such as:

nationalities ending with -i like: Irani, Iraqui, Azerbaijani etc.
ethnic groups: Blackfeet, Crows, Iroquois, Seneca, Beduins, Tuaregs etc.
non-existent groups: Vikings, Picts, Normans, Saxons, Cimbri, Greeks (ancient), Romans (ancient), Incas, Victorians, Elizabethans, American Pilgrims etc.
ideological groups: communists, bolsheviks, republicans, conservatives, nazis, labourers, wigs, torries, impressionists (french impressionists), surrealists, pre-raphaelists, stoics etc.
religious groups: muslims, christians + carmelites, madeleines, templars, puritans, sikhs, talibans etc.
mythological groups: titans, cyclops, amazons (non-counted groups unlike the muses, the erinyes)