It should work but you should be extra careful about weight distribution and turning. Since the front and back wheels are a different distance apart, they have different radii of turning. AKA the back wheels will travel further than the front wheels will in the same amount of time, straining the back wheels’ motors harder.
Yes it should be possible to make this work, but there are a couple of things you should consider:
Depending on how the robot is set up (where the centre of mass is, and the exact spacing of the wheels), the more closely spaced pair of wheels may turn more slowly when the robot rotates than the more widely spaced wheels. If that happens then the more closely spaced wheels will be carrying more of the load. The closely spaced wheels may even end up forcing the widely spaced ones to turn faster than their free speed (i.e. the closely spaced wheels might backdrive the widely spaced ones). This is a situation you want to avoid. Putting the weight towards the wide end of the robot should help prevent these problems.
The robot will probably travel in an arc while strafing, at least while it accelerates. This would happen if the wheels exert different torques on the robot as a result of being different distances from the centre of mass. Again, moving the weight towards the wide end ought to help.
Forward and backwards movement will be the same as with an ordinary mecanum drive.
Motors produce more torque and draw more current when they are turning slowly than when they are turning quickly. This means it is the wheels that move less far that you need to worry about.
I meant for a constant voltage. Reducing the power value reduces the voltage across the motor which reduces both the maximum torque and the maximum speed.
For a constant voltage though, torque decreases as speed increases. So yes, if you force a motor to move slowly (including by putting a high load on it, like making it move a heavy robot or lift a heavy arm) it will exert more torque and draw more current than it would if it were unloaded.
A really good image to illustrate this is this one:
Decreasing the motor’s power value in code will just pull that white line downwards.
(The reason why current drops off as motor speed increases is because of the way both motors and generators work. If you apply a voltage, and thereby a current, to a coil in a magnetic field then the coil will move. This is a motor. If you move a coil through a magnetic field it induces a voltage in the coil. This is a generator. So motors and generators are similar things used in different ways. You can use a motor as a generator by completing the circuit across its pins and manually turning it. In a motor, once the coil starts to move through the magnetic field a voltage is induced in it, as in a generator. This voltage, which is called a back EMF*, opposes the supply voltage, which is causing the coil to turn. The back EMF partially cancels out the supply voltage and means that there is less current through the motor than there would be if there were no back EMF (which is the case when the motor is not moving, i.e. stalled). The faster the motor turns, the more back EMF, and the less current. When the coil is turning fast enough that the supply voltage and the back EMF are equal and opposite, they cancel and the current is zero.
*EMF means electromotive force and in this context just means a voltage induced by movement through a magnetic field. It is measured in volts because it is a voltage, not a force, it just has a confusing and illogical name. See http://en.wikipedia.org/wiki/Back_EMF)