Section 21. Indebtedness authorized under section twenty shall be outside the limit of indebtedness prescribed in section ten of chapter forty-four, and shall be payable within twenty years and otherwise subject to sections sixteen to twenty-seven, inclusive, of said chapter forty-four, except that in the case of indebtedness authorized under clause (1) or clause (2) of section twenty, the first principal payment may be made within five years and the last within twenty-five years of the date of the bonds or notes issued for the Serial Loan. The total amount of indebtedness of any city or town outstanding at any one time under clause (3) of section twenty alone shall not exceed one half of one per cent, and under clauses (1), (2) and (4) combined of said section twenty shall not in any city with a population over one hundred and fifty thousand exceed seven per cent of its equalized valuation and in any other city exceed six per cent of its equalized valuation.