I've polished/color sanded some metallics with great success, so don't anyone let you tell you that it can't be done. Generally the larger the flake, the more difficult to impossible it will be. I've done Duplicolor and Tamiya sprays, but usually clear over them after.
I often make up "sample" parts to experiment on, so I tend to try a lot of things out and not on the actual model. Success teaches us nothing.
I believe that the smoother the finish coat is, the smoother the clear will lay down the less of a chance you'll blow through the clear into the color when you polish that.
I don't necessarily buy into the Tamiya "one month" myth, everything I paint goes right into the dehydrator.
Bob