1. All wage-earning Imperial citizens.
Quoting from the Wikia (emphasis mine):
Under Republic law, all wage-earning beings were required to pay income tax. This tax was collected by the Republic Tax Collection Agency. When the Trade Federation complained about piracy in the outlying star systems, the Republic Senate passed a tariff on those formerly free trade zones. This action justifiably upset the Trade Federation Viceroy, prompting him to blockade and invade the small planet of Naboo. Right before the Clone Wars, a fifteen percent transit tax was levied on all tickets sold in an effort to raise emergency funds for the government. After the Galactic Republic changed to the Galactic Empire, tax laws were consolidated into the Imperial Revenue Codes and placed under control of the Imperial Taxation Bureau.
The Republic Tax Collection Agency was replaced under the New Order by the Imperial Taxation Bureau, which:
was the government body under Emperor Palpatine that was responsible for collecting taxes from Imperial citizens, as well as setting tax rates.
As for question 2, the only relevant quote I've found is the fifteen percent one highlighted above.