Controlling the individual speed of mecanum wheels within the main programme

[magnets]

The source for wpilib is included as a project in Java, but I don't know if it's included in the C++ release.
If you want to right your own mecanum code, it's pretty easy. Here's the important part from the Java library.

 frontLeft = xIn + yIn + rotation;
          frontRight = -xIn + yIn - rotation;
        rearLeft = -xIn + yIn + rotation;
        rearRight = xIn + yIn - rotation;

Just make sure that you invert your y axis on the joystick. Also, you should have you code check to see if any of the wheels speeds are > 1, and if so, divide them all by the largest value present to make sure that you still get the right response.

As for the wheel speed issue, it can definitely help to use closed loop control for the velocity of the wheels, but it can be fairly difficult to implement well, and it will take a long time to tune it well. I would recommend just sticking with the simpler method unless your robot really isn't controllable. If it is, make sure there's no excess friction in any gearboxes/drive components/rollers on the wheels.