WASHINGTON (Reuters) - A new Internal Revenue Service unit set up to catch rich tax cheats hiding their wealth in complex business entities is rapidly taking shape with the hiring of hundreds of employees..... President Barack Obama has proposed tightening tax rules for U.S. multinationals, including one in which companies delay paying taxes on income earned offshore, a legal practice known as deferral that officials say is abused.... "With any new thing, you never want to be the guinea pig," Mary Lou Fahey, general counsel for the Tax Executive Institute, comprised of business executives, said.