I am not familiar enough with the straight forward way to scale import models. However, I do know you can trick the imported parts by using the Units settings in File>Prepare>Model Properties>Units. You can make a new unit as .66 or 2/3 if you want more accuracy. Now create a new System of Units to include your new units. When you change your part to this new unit system setting, you have the option of scaling the model units. once this is done, you can revert back to your desired units by using the other conversion method.
A little effort will get you want you need. It is the only way I know to fool Creo to get what you want.