Brand new brakes and rims need a bit of bedding in. You shouldn't do any safety critical braking until you have got rid of any surface gunge on the rim and pads. I usually find a steady hill and ride down with the brakes rubbing, making sure I dont go too fast.
With any new bike, you need to guage the braking capacity before you use them in anger. The best solution for brakes which are too grabby, if the front end starts to lose traction or shimmy is to release the front brakes, keep applying the rear, and try again.