Fixed income is simply a investment classification based primarily on how you earn a return.

Seniority and placement on the balance sheet of debt and equity issues from an accounting or bankruptcy point of view is an entirely different matter.

Preferred shares are often considered to be fixed income or a proxy for fixed income for the simple reason that up until the last 5 - 10 years most issues paid a fixed dividend.  Further, historically most issues were cumulative and had relatively firm terms as to when the issue would be redeemed.  i.e. very bond like.

In my opinion most new preferred issues have given up the bond like qualities that made them attractive in the first place.