Dear Mr. Vasant,
A private company can issue unsecured CONVERTIBLE Debenture to any no. of persons otherwise the same will be regarded as "Deposit"
A private company can issue unsecured Debenture to its shareholders/directors and their relatives, otherwise again the same will be regarded as "Deposit"
A private company can issue pref shares subject to conditions/provisions of Section 80 of the Companies Act, 1956 and the rate of dividend may be from 0% to any reasonable rate, however the same can be paid on availability of profits only.
Preference shareholders can't be counted as memers for the limit of 50 as prescribed under section 3(1)(iii) of the Cos. Act,
CS Ashwini Kumar