Hi Fabio,
What you're describing is not a monopole system. The iron induces an opposite polarity to the rotor magnets. Even with all opposing fields having the same polarity, it would still only be unipolar. Monopole means there is a single magnetic polarity present in the system.
There is such a thing as a monopole motor. Edwin Gray had a motor which was tested by an independent laboratory which found that the motor had a 360 degree North Pole magnetic field. Both ends of all coils were North Pole, as well as the SIDES of the coils. This motor also had an efficiency of 25,000 percent. It produced an output of ten HP, under load, with an input of just under 28 Watts.
And it's easy enough to build this kind of system - the key is to use magnetic current, rather than normal electricity. With North Pole current, the only fields you'll have will be North Pole. Gray copied Tesla's Radiant Energy system to produce the magnetic current. (What I call diamagnetic energy). EMF is combined with CEMF by discharging a capacitor through a spark gap, in series with an inductor. The inductor side of the gap produces North Pole current, while the capacitor side produces South Pole current.
Tesla used very short pulses, so that the energy would radiate outwards, perpendicular to the gap and wires. But a motor requires longer term pulses. Gray knew that CEMF exists only as long as the EMF continues to increase. So he constructed his pulses by sequentially discharging three or more capacitors having increasing potentials. This produces a stair step waveform which simulates a triangle wave lasting for the duration of the pulse. A long baseline triangle wave can also be produced by using a capacitance multiplier circuit to adjust a capacitor's reactance.