In order to encourage developments which are not economically homogeneous and to achieve rent charges which will make units available to persons and families of low income at low rentals and moderate income at moderate rentals the agency and a mortgagor may use measures including, but not limited to: direct rental assistance in the form of partial rent subsidy from any county, municipal, Commonwealth or Federal source, allocation of lower rents, and the raising of rents in the majority of apartments in the project in order to lower the rents of those in the lower rent charge category. With respect to each project the agency shall, prior to initial occupancy, allocate and prescribe the number of lower rental units and the rents to be charged therefor. The allocation may be reviewed and adjusted from time to time. The method of achieving lower rental charges shall, in each instance, be prescribed by the agency.