Section 20C. The district attorneys may each appoint and may remove at pleasure such additional attorneys-at-law as special assistant district attorneys as may be necessary to participate in federally funded demonstration programs under the Omnibus Crime Control and Safe Streets Act of 1968 or other federal acts; provided that the compensation of such special assistant district attorneys shall be paid from federal funds. Such special assistant district attorneys shall have all the powers of assistant district attorneys and shall receive such salary not exceeding that provided in section sixteen for assistant district attorneys in their respective districts as the district attorney may determine.